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READ THIS BEFORE PRINTING!!!! This document has been formated in a special way. Virtually all dot matrix printers have a condensed mode which prints 132 characters across a standard 8 1/2 inch page. When this file is printed out in condensed mode, the resulting printed pages can be cut down to 5 1/2 X 8 1/2 inches. The cut pages will fit nicely in the back of your DOS manual for storage. Typically, you can turn on this mode by sending a special control sequence to the printer from BASIC. For example, you can turn on the condensed mode of the IBM/Epson printer with the BASIC statement: LPRINT chr$(15). If your printer has such a condensed mode, turn it on now, before printing the rest of this document. (tm) CHASM Cheap Assembler for the IBM Personal Computer User's Manual (c) 1983 by David Whitman Versions 2.10/3.10 David Whitman 2 North Park St. Apartment L Hanover, NH 03755 Table of Contents I. Why CHASM?...............................................2 II. What can CHASM do?......................................3 III. What WON'T it do?......................................3 IV. Setting Up a CHASM Work Disk............................4 V. Modifying CHASM's I/O Defaults...........................6 VI. Syntax..................................................9 VII. Operands..............................................11 VIII. Resolution of Ambiguities............................15 IX. Pseudo-Operations......................................18 X. Outside the Program Segment.............................21 XI. Running CHASM..........................................22 XII. Error and Diagnostic Messages.........................24 XIII. Execution of Assembled Programs......................27 XIV. Notes for Those Upgrading to This Version of CHASM....31 XV. Miscellaneous and A Word From Our Sponsor..............34 XIV. Compiled Version Order Form...........................37 2 I. Why Chasm? Why go to the trouble to write an assembler, when one already exists? The IBM Macro Assembler is a very powerful software tool available off the shelf. It supports features such as macros, definition of multiple segments, and linking to external procedures. Unfortunately, all of this power doesn't fit into a 64K machine, and even when using the small subset version, 64K users are limited by memory to only very small programs. The macro assembler is also very complex, hard to understand, and costs a hundred bucks. Even though the price of memory keeps dropping, I suspect that the majority of the IBM PC's out there have no more than 64K installed. Also, I suspect that most end-user assembly language programmers are like myself, and are not interested in writing huge, complicated programs in assembler. I want to write short subroutines to call from BASIC, small patches to existing assembler programs (such as DOS), and perhaps some games. For such uses, I think the combination of the Macro Assembler and a tub full of extra memory represents an incredible overkill. Chasm is, I hope, a more reasonable compromise between power and accessibility (both in cost and complexity). A Note for Beginners: CHASM is much easier to use than the IBM macro assembler, and is ideal for learning assembly language. If you consider yourself a beginner, you might find it useful to print out and read the file PRIMER.DOC which is included on your CHASM distribution disk. PRIMER is a gentle introduction to assembly language programming, which will teach you some of the vocabulary and key concepts you will need to start out with. 3 II. What can Chasm do? Chasm takes a text file, consisting of mnemonics, user-defined symbols, numbers, and pseudo-ops, and produces a file of corresponding machine language for the 8088 processor. Chasm allows you to define labels for branching, rather than requiring you to figure out offsets or addresses to jump to. It allows you to represent with a name any constants you want to use, making your programs easier to understand. Most importantly, it translates mnemonics to their machine language equivalents freeing you from the task of hand translation. III. What WON'T it do? In the interest of simplicity, Chasm has a number of restrictions: 1. Statement syntax is not quite as free as in the macro assembler. 2. The number of pseudo-ops is severely cut down from the macro assembler. 3. Macros are not supported. (Note that the IBM assembler doesn't support macros in systems smaller than 96K) 4. Expressions (such as BUFFER - 2 ) are not supported, at least in version 1.0. 5. Multiple segment definitions are not allowed, at least in version 1.0. Chasm assumes that your entire program fits in one segment, that the cs, ds, and es registers all point to this same segment, and that the ss register points to a valid stack area. 6. External linking is not supported, at least in version 1.0. 4 IV. SETTING UP A CHASM WORK DISK CHASM comes in two flavors: interpreted and compiled. You can tell which version you have by listing the directory of your CHASM distribution disk. If the file CHASM.EXE is present, you have the compiled version. If not, you have the interpreted version. Follow the appropriate instructions below to make a CHASM work disk: Compiled version: Format a fresh diskette with the /S option (to include DOS). Now copy the following files from your CHASM distribution disk: CHASM.EXE CHASM.DAT Interpreted version: Format a fresh diskette with the /S option (to include DOS). From your master DOS diskette, copy the file: BASIC.COM Now copy the following files from your CHASM distribution disk: CHASM.BAT CHASM.BAS CHASM.DAT CHASM.OVL Both versions: The file CLEAR.ASM on your CHASM distribution disk is provided as a sample assembly language source file. If you want to try out CHASM before writing a source program of your own, copy CLEAR.ASM onto your work disk. If you wish to modify any of CHASM's I/O defaults, you should include a CHASM.CFG file on your work disk. See section VII for a discussion of the CHASM.CFG file. You may find it convenient to keep a copy of your text editor (EDLIN, VEDIT, etc.) on your work diskette, for creation and modification of source files. 5 If you plan to incorporate machine language routines in BASIC programs, copy the utility program COM2DATA (file COM2DATA.EXE or COM2DATA.BAS) from your CHASM distribution disk. 6 V. MODIFYING CHASM'S I/O DEFAULTS CHASM supports the use of a configuration file, which can be used to override some of CHASM's default assumptions about your printer and video monitor. If you are willing to accept CHASM's defaults, you may skip this section - CHASM will work perfectly well without the file described below. The configuration process involves supplying a file named CHASM.CFG on your work disk. The file should contain a series of text "switches" of the following form: /switch, nn, [mm, oo,...] Where "/switch" is a reserved word, listed below, and nn, mm, oo are numbers. The brackets around mm and oo indicate that these numbers are optional - don't put brackets in CHASM.CFG. Each number or word should be separated by a comma or should start a new line. The following switches are implemented: /132 Printer 132 column mode. The numbers following this switch are the ASCII codes for the characters which cause your printer to go into condensed mode. You may specify as many characters as you like. If you don't provide this switch, CHASM will truncate source lines in listings to avoid going over 80 columns. Example: (for IBM printer) /132, 15 /80 Printer 80 column mode. Similar to /132, but the numbers represent the characters to return your printer to normal. Example: (IBM printer again) /80, 18 7 /BEEP Enables/Disables audible warning when errors are discovered in your source program. A value of zero turns off beeping, anything else turns it on. Example: (off) /BEEP, 0 /FG Foreground color. Users with color monitors may select a foreground color from the following list: 0 Black 8 Gray 1 Blue 9 Light Blue 2 Green 10 Light Green 3 Cyan 11 Light Cyan 4 Red 12 Light Red 5 Magenta 13 Light Magenta 6 Brown 14 Yellow 7 White 15 High Intensity White Example: (Magenta) /FG, 5 /BG Background color. Selections 0-7 above are available. Example: (Cyan) /BG, 3 /PAGELEN CHASM assumes that there are 66 lines to each printed page. If you use non-standard paper, enter the appropriate number after this switch. Example: /PAGELEN, 72 /LINES By default, CHASM will print 58 lines on each printed page, then skip over the perforation to the next page. You can change this number to suit your paper and personal taste. Example: /LINES, 50 A sample CHASM.CFG file, suitable for use with the IBM dot matrix printer, is included on your CHASM distribution disk. 8 Note for users of Jim Button's PC-FILE (tm): CHASM.CFG has the same format as PC-FILE's PRT.CTL file, except that the /40 switch is not implemented. If you have already written a special PRT.CTL file, just make a copy named CHASM.CFG for use with CHASM. 9 VI. Syntax Chasm accepts a standard DOS text file for input. Lines may be any combination of upper and lower case characters. Chasm does not distinguish between the two cases: everything except single quoted strings are automatically converted to upper case during the parsing process. Thus, BUFFER, Buffer, buffer, and bUFFer all refer to the same symbol. The characters blank ( ), comma (,), single quote (') semi-colon (;) and TAB are reserved, and have special meaning to Chasm (see below). Note also that Chasm is written in BASIC, and use of the double quote character (") may cause bizarre effects by confusing the interpreter into breaking down input lines in an undesirable way. The double quote may be safely represented in a text message by declaring a byte containing its ASCII value. Thus, to represent the text: Use ASCII value for "double quotes" You may use: DB 'Use ASCII value for ' 22H 'double quotes' 22H Each line must be less than 256 characters long and have the following format: Label Operation Operand(s) ;comment The different fields of an input line are separated by the delimiters blank ( ), comma (,) or TAB. Any number of any delimiter may be used to separate fields. Explanation of Fields: Label: A label is a string of characters, beginning in column 1. Depending on the operation field, the label might represent a program location for branching, a memory location, or a numeric constant. Note that anything beginning in column 1, except a comment, is considered a label. 10 Operation: Either a pseudo-op (see section IX) or an instruction mnemonic as defined in "The 8086 Book" by Rector and Alexy. Note 1: Except as modified below,"The 8086 Book" is the definitive reference for use with Chasm. Note 2: There are several ways to resolve some ambiguities in 8086 assembly language. Please read page 3-285 of The 8086 Book, and section VIII of this document. Operand(s): A list of one or more operands, as defined in section VI, separated by delimiters. Comment: Any string of characters, beginning with a semicolon (;). Anything to the right of a semicolon will be ignored by Chasm. Note that except for the case of an operation which requires operands, or the EQU pseudo-op which requires a label, all of the fields are optional. The fields MUST appear in the order shown. 11 VII. Operands The following operand types are allowed. 1. Immediate data: A number, stored as part of the program's object code. Immediate data are classified as either byte, expressible as an 8 bit binary integer; or word, expressible as a 16 bit binary integer. If context requires it, CHASM will left-pad byte values with zeroes to convert them to word values. Attempts to use a word value where only a byte will fit will cause an error message to be printed. Immediate data may be represented in 5 ways: A. An optionally signed decimal number in the range -32768 to 32767. Examples: MOV AL,21 MOV BX,-6300 B. A series of up to 4 hex digits, followed by the letter H. The first digit must be non-alphabetic, i.e. in the range 0-9, to allow CHASM to distinguish between numbers and symbols. If necessary, a leading zero, which does not count in the four allowed digits, may be added to fufill the non-alphabetic condition. Examples: ADD CX, 0B123H ADD DL, 12H C. A symbol representing types A or B above, defined using the EQU pseudo-op. Examples: MASK EQU 10H MAX EQU 1000 AND CL,MASK SUB AX,MAX 12 D. The offset of a label or storage location returned by the OFFSET operator. OFFSET always returns a word value. OFFSET is used to get the address of a named memory location, rather than its contents. Example: MOV DI,OFFSET(BUFFER) BUFFER DS 0FFH E. The ASCII value of a printable character, represented by the character enclosed in single quotes ('). Thus, the following lines will generate the same object code: MOV AL,41H ;ascii code for 'A' MOV AL,'A' 2. Register Operands: One of the 8088's internal registers. A. An 8 bit register from the following list: AH AL BH BL CH CL DH DL B. A 16 bit register from the following list: AX BX CX DX SP BP SI DI C. A segment register from the following list: CS SS DS ES 3. Memory Operands: The contents of a memory location addressed by one of the following methods. Note that none of the memory addressing options specifies whether a byte or word operand is being referenced. See section VIII for more on this topic. A. Direct address. 1. A number, or symbol representing a number, enclosed in brackets, indicating an offset into the data segment. Example: BUFFER EQU 5A5AH MOV BH,[BUFFER] MOV [80H],DI 13 2. A symbol, defined to be a variable (i.e. a named memory location) using the EQU pseudo-op. Example: FCB EQU [80H] MOV DI,FCB 3. A symbol, defined to be a variable by its use on a storage defining pseudo-op. Examples: MOV AX,FLAG MOV DATE,BX FLAG DS 1 DATE DB 31 B. Indirect Address: The address of the operand is the sum of the contents of the indicated register(s) and a displacement. The register, or sum of registers, are enclosed in square brackets: [] The displacement is optional, and takes the form of an immediate operand, placed without intervening delimiters to the left of the first bracket. Displacements in the range -128 to 127 (i.e. hex 0 - 7F, FF80 - FFFF) are interpreted as signed 8 bit quantities. All other displacements are interpreted as unsigned 16 bit quantities. (Note that although the 8088 supports unsigned 16 bit displacements up to hex FFFF for indirect addressing, Chasm isn't smart enough to distinguish between -1 and FFFFH.) The following indirect modes are allowed: 1. Indirect through a base register (BX or BP). Examples: ENTRYLENGTH EQU 6 MOV AX, ENTRYLENGTH[BP] MOV DL, -2[BX] MOV CX, [BP] MOV 9A9AH[BX], AX 2. Indirect through an index register (DI or SI). Examples: MOV [DI], CX MOV CX, -5[SI] 14 3. Indirect through the sum of one base register and one index register. Examples: MOV [BP+DI], SP ;note that no spaces are MOV BX, 10H[BX+SI] ;allowed within the MOV CL, [BP+SI] ;brackets. MOV DH, -2[BX+DI] 4. Labels A label on an instruction may be used as an operand for call and jump instructions. Examples: START PROC NEAR CALL GETINPUT JMPS START ENDP GETINPUT PROC NEAR 5. Strings A string is any sequence of characters (including delimiters) surrounded by single quotes ('). Example: DB 'Copyright May 15,1982' 15 VIII. Resolution of Ambiguities. The language defined in "The 8086 Book" contains a number of ambiguities which must be resolved by an assembler. This is discussed throughout the book, but pages 3-285 and 3-286 specifically cover this topic. Chasm's solutions of these problems are discussed in this section. A. Memory references: When one specifies the address of a memory location, it is unclear how large an operand is being referenced. An operand might be a byte, or a word. 1. If a register is present as an operand, it is assumed that the memory operand matches the register in size. An exception to this rule are the shift and rotate instructions, where the CL register is used as a counter, and has nothing to do with the size of the other operand. Examples: MOV MASK,AX ;mask is a word MOV DH,[BX] ;BX points to a byte NEG [SI] ;error, operand of unknown size SHR FLAG,CL ;error, flag is of unknown size 2. If no register is present, (or if the only register is CL being used as a counter) the size of the memory operand is specified by adding the suffix "B" or "W" to the instruction mnemonic. Examples: NEGB [SI] ;SI points to a byte SHRW FLAG,CL ;flag is a word MOVW MASK,0AH ;mask is a word MOVB MASK,0AH ;mask is a byte MOVW MASK,9A9AH ;must specify size even though ;immediate operand implies word MOVB DH,[BX] ;error(!), register already ;specifies size 16 B. Indirect Branching. The 8088 supports two flavors of indirect branching: intra, and intersegment. A register is set to point at a memory location which contains a new value for the program counter, and in the case of intersegment branching, a new value for the CS register as well. The syntax of "The 8086 Book" does not specify which flavor of branch is being invoked. Chasm adds the suffixes "N" (for near, or intrasegment) and "F" (for far, or intersegment) to the indirect CALL and JMP mnemonics. Examples: CALLN [BX] ;intrasegment call JMPF [DI] ;intersegment jump JMP [BP] ;error, unspecified flavor C. Long and Short Jumps Two types of relative jumps are supported by the 8088: short (specified by a signed 8 bit displacement) and long (specified by a 16 bit displacement). Both are implemented in Chasm as a jump to a label. The short jump is specified by mnemonic JMPS, in accord with the IBM disassembler, but not with The 8086 Book, which uses JMP. Since one of the displacement bits is used to indicate direction, only seven are left to express the magnitude of jump. JMPS (and similarly, all the jump on condition instructions) is thus limited to branching to labels within a range of -128 to +127 bytes. Chasm reserves mnemonic JMP for the long jump. JMP may be used to jump anywhere within the program segment, but the object code generated is less compact than that from JMPS. Examples: START PROC NEAR JMPS END ;short jump JMP START ;long jump END ENDP 17 D. Instruction Prefixes. The 8088 supports three instruction prefixes: 1. SEG: segment override. An alternate segment register is specified for a reference to memory 2. REP, REPE,REPNE,REPZ,REPNZ: repeat. A string primitive is repeated until a condition is met. 3. LOCK: Turns on the LOCK signal. Only useful in multiprocessor situations. Chasm implements these prefixes as separate instructions, rather than prefixes to another instruction. They appear on a separate line, immediately before the instruction which they modify. This is in accord with the output of the IBM disassembler, but not with the IBM macro assembler. Examples: SEG ES MOV AX,FLAG ;flag is in the extra segment REP MOVSB ;move bytes until CX decremented to 0 18 IX. Pseudo-Operations The following pseudo-ops are implemented: A. BSAVE: Generate object code in BSAVE format. Instructs CHASM to build a header in the format of BASIC's BSAVE command. The resulting object code file may be BLOADed by BASIC programs. No operands are required, and the pseudo-op may appear anywhere within the source code. Example: ORG 0 ;no psp SUBRT PROC FAR ;subroutine for BASIC program BSAVE ;make BLOADable object file B. DB: Declare Bytes Memory locations are filled with values from the operand list. Any number of operands may appear, but all must fit on one line. Acceptable operands are numbers between 0 and FFH (0-255 decimal), or strings enclosed in single quotes ('). If a label appears, it is considered a variable, and the location may be referred to using the label, rather than an address. Examples: MOV AX,MASK MASK DB 00H,01H STG DB 'A string operand' C. DS: Declare Storage Used to declare large blocks of identically initialized storage. The first operand is required, a number specifying how many bytes are declared. If a second operand in the form of a number 0-FFH appears, the locations will all be initialized to this value. If the second operand is not present, locations are initialized to 0. As with DB, any label is considered a variable. To save space, the object code does not appear on the listing. Examples: DS 10 ;10 locs initialized to 0 DS 100H,1AH ;256 locs initialized to 1AH 19 D. EJECT: Begin New Print Page Causes CHASM to move to the top of the next page in printed listings. Has no effect on listings sent to the screen or to disk. E. ENDP: End of Procedure See PROC (below) for details. F. EQU: Equate Used to equate a symbolic name with a number. The symbol may then be used anywhere the number would be used. Use of symbols makes programs more understandable, and simplifies modification. An alternate form of EQU encloses the number in square brackets: []. The symbol is then interpreted as a variable, and may be used as an address for memory access. This version is provided to allow symbolic reference to locations outside the program segment. Examples: MOFFSET EQU 0B000H MONOCHROME EQU [0000H] Warning: Difficult to debug errors may result from using a symbol prior to its being defined by EQU. I strongly urge that all equates be grouped together at the beginning of programs, before any other instructions. See "Phase Error" in section XII. G. ORG: Origin Allows direct manipulation of the location counter during assembly. By default, Chasm assembles code to start at offset 100H, thus leaving room for the program segment prefix normally built by COMMAND or DEBUG. In situations where no PSP is provided, such as routines to be called from BASIC, you should override this default with ORG, or incorrect assembly may result. 20 ORG requires one operand, a number between 0 and 32767, which represents the new setting of CHASM's location counter. Although the location counter may be reset anywhere within a program, generally this pseudo-op should be used before any machine executable instructions for meaningful results. Example: ORG 0 ;Code will be assembled for starting ;offset of 0 H. PROC ...ENDP: Procedure Definition Declares a procedure. One operand is required on PROC, either the word NEAR, or the word FAR. This pseudo-op warns Chasm whether to assemble returns as intra (near) or intersegment (far). Procedures called from within your program should be declared NEAR. All others should be FAR. ENDP terminates the procedure, and requires no operands. If a RET is encountered outside of a declared procedure, an error occurs. Procedures may be nested, up to 10 deep. Example: MAIN PROC FAR ... ... ;body of procedure ENDP 21 X. Outside the Program Segment As mentioned previously, Chasm does not support multiple segment definitions. Provision is made for limited access outside of the program segment, however. A. Memory References To access memory outside the program segment, you simply move a new segment address into the DS register, then address using offsets in the new segment. The memory option of the EQU pseudo-op allows you to give a variable name to offsets in other segments. For example, to access the graphics character table in ROM: BIOS EQU 0F000H CHARTABLE EQU [0FA6EH] MOV AX,BIOS ;can't move immed. to DS MOV DS,AX MOV AL,CHARTABLE ;1st byte of char. table B. Code Branching Chasm supports 4 instructions for branching outside the program segment. 1. Direct CALL and JMP New values for the PC and CS registers are included in the instruction as two immediate operands. Example: BIOS EQU 0F000H ;RAM bios segment DISKETTE_IO EQU 0EC59H ;disk handler JMP DISKETTE_IO,BIOS 2. Indirect CALLF and JMPF Four consecutive bytes in memory are initialized with new values for the PC and CS registers. The CALLF or JMPF then references the address of the new values. Example: BIOS EQU 0F000H ;RAM bios segment PRINTER_IO EQU 0EFD2H ;printer routine MOV [DI],PRINTER_IO MOV 2[DI],BIOS CALLF [DI] 22 XI. Running Chasm From DOS, type: CHASM After a few seconds, Chasm prints a hello screen, then prompts: Hit any key to continue... Chasm prompts: Source code file name? [.asm] Type in the name of the file which contains your program. If you do not include an extension for the filename, Chasm assumes it to be .ASM. If Chasm is unable to find the file, it will give you the option of naming another file, or returning to DOS. Assuming your file is present, Chasm prompts: Direct listing to Printer (P), Screen (S), or Disk (D)? Respond with either an upper or lower case letter. If you select "D", Chasm will prompt: Name for listing file? [fname.lst] Type in a name for the listing file. If you just press ENTER, the name defaults to that of your source file, with an extension of .LST. The final prompt is: Name for object file? [fname.com] Type in a name for the assembled program. If you just press ENTER, the name defaults to that of your source file, with an extension of .COM. 23 Chasm now assembles your program. A status line is maintained on the screen, showing how many lines have been processed. Chasm makes two passes over your source file, outputting the listing and object code on the second pass. Be patient, Chasm is written in BASIC, and does a good imitation of a snail crawling through molasses. ****NOTE: I will send the Golden Bootstrap Award to the first person submitting a complete implementation of Chasm, rewritten in assembly language, and small enough to be assembled by Chasm version 1.9 on a 64K system. When finished, Chasm returns you to DOS, and you see the prompt: A> 24 XII. Error and Diagnostic Messages Error messages generated on pass one appear on the listing before any source code is printed, and mention the line number to which they refer. The majority of messages occur during pass two, and will appear in the listing immediately prior to the line which caused the message. Add Leading Zero to Hex Constant: Diagnostic. The Unrecognized Symbol could be interpreted as a hexadecimal number if a leading zero was added. Could Use JMPS: Diagnostic. The specified label requires an offset of less than 128 bytes; specifying the short jump would result in more compact code. The assembled code is correct, however. Duplicate Definition of XXX in (linenum): Pass 1 error. An attempt was made to define a symbol already present in the symbol table. ENDP without PROC: An ENDP was encountered, but no corresponding PROC was found. EQU Without Label: No symbol was found to equate with the operand. Illegal or Undefined Argument for Offset: The argument for the Offset function was not present in the symbol table as a near label or memory location on pass 2. Missing ENDP: The end of the input file was encountered, and at least one PROC had not been terminated by an ENDP. Phase Error: A near label or memory location is found to have different values on pass 1 and pass 2. A difficult to debug error: generally the problem is not caused by the statement which received the error message. 25 The only documented way to generate this error is by using a symbol prior to defining it using EQU. On pass 1, Chasm assumes that unrecognized symbols are either labels or memory addresses which will be defined later in the program. If the symbol is subsequently defined as immediate data, the location counter may get messed up. I would appreciate hearing about any other situations which cause this message to appear. Procedures Nested Too Deeply: Procedures may be nested no more than 10 deep. Specify Word or Byte Operation: Diagnostic. Chasm suggests that the Syntax Error might be resolved by adding the suffix "B" or "W" to the instruction mnemonic. Most, but not all, ambiguous memory references are flagged with this diagnostic. Syntax Error: (OP) (DTYPE) (STYPE). Chasm was unable to find a version of the instruction (OP) which allows the operand types (DTYPE) and (STYPE). Either the instruction doesn't exist, or it is an inappropriate choice for the given operands. DTYPE and STYPE are numbers which encode Chasm's guess as to what the 1st and 2nd operands might be. The numbers are sums of the types given in the following list: 1 = 8 bit accumulator 2 = 16 bit accumulator 4 = 8 bit register 8 = 16 bit register 16 = indirect memory reference 32 = CS register 64 = other segment register 128 = memory location 256 = immediate byte 512 = immediate word 1024 = none 2048 = string 4096 = near label 8192 = reserved 16384 = CL (count) register Too Far For Short Jump: The displacement to the specified label is not in the range -128 to +127. Too Many User Symbols in (linenum): Pass one error. The symbol table is full. Undefined Operand for EQU: Any operands on an EQU statement must have been previously defined. Undefined Symbol XXX: The symbol XXX was used as an operand, but never appeared as a label, and is not a predefined symbol. 26 Unrecognized Operand XXX: XXX is used in the DB operand list, but is not a string or byte value. 27 XIII. Execution of Assembled Programs A. Object code format The object code file produced by CHASM is in the form of a memory image, exactly as will be present in your computer at run time. No link step is required. Provided that the segment registers are set correctly, the architecture of the 8088 guarantees that code is self-relocating, and will run correctly loaded anywhere in memory. Storing a program as an exact image of memory at run time is called the COM format by IBM. This COM format is *not* that produced by the IBM assembler. The output of the IBM assembler is in the form of an "object module" suitable for input to the linker. To the best of my knowledge, the object module is not directly executable, but must first be "filtered" through the linker. This adds an extra step to the process of producing a working program, but gives you the option of combining multiple object modules into one program. The resulting linked program is *still* not a memory image, but has a header which is used to perform relocation during loading. This linked program plus header is called the EXE format by IBM. B. Running Assembled Programs From DOS DOS provides a loader for running machine language programs. To run a program, you merely type its name, with or without the extension. This is what you're doing every time you use a DOS external command such as FORMAT or CHKDSK. In fact, the COM format is named after "external COMmand". When you type the name, DOS examines the file extension to determine what format the file is in, either COM or EXE. This is why CHASM defaults to using the extension .com for your object file. If you plan to run the program from DOS, don't change the extension. For COM programs, DOS builds a 255 byte long "program segment prefix" and sets the segment registers to point to this PSP. The contents of the file are then loaded verbatim right after the PSP, at offset hex 100 in the segment defined by the segment registers. As soon as loading is complete, your program is executed starting with the instruction at hex 100. 28 Although you can totally ignore the PSP, you should read pages E-3 through E-11 of the DOS manual to see what DOS puts there for you. It turns out there are some real goodies which your program might want to use. When your program is done, it must transfer control back to DOS, otherwise the 8088 will continue to fetch what it believes are instructions from whatever garbage or bit-hash happens to follow your program in memory. The easiest way to return to DOS is to execute the instruction: INT 20H This is the vectored interrupt reserved by DOS for program termination. While we're on the topic of vectored interrupts, you would be well rewarded to study both the DOS manual and Technical Reference to find out what happens when you execute some of the other interrupts. Some very useful functions, such as file handling and screen i/o, are available at the machine language level through this mechanism. Looking at things the other way, by changing the interrupt vector for a given function to point to your own code, you can override the way DOS or the BIOS does something, and do it your way. DOS even provides a method (via interrupt 27H) by which your new code can be grafted onto DOS, and not be overwritten by other programs. C. Debugging Assembled Programs IBM provides an excellent utility with DOS, called DEBUG.COM. By specifying your program's name as a parameter when invoking DEBUG, you can observe your program execute with DEBUG's trace and other functions. To debug your program, from DOS type: DEBUG progname.COM DEBUG builds a PSP and loads your program just like DOS does, but you have the added power of the debugging commands to monitor your program while it runs. See chapter 6 of the DOS manual for more details about using DEBUG. 29 D. Using Assembled Programs in BASIC To incorporate a machine language subroutine in a BASIC program, write it in assembly language, then assemble it with CHASM. You should read page C-7 of the BASIC manual for some conventions to use in writing your subroutine. In particular, note that you must declare the routine to CHASM as a FAR procedure using the PROC pseudo-op, and that the last instruction of the routine should be a RET. Unlike programs which are run directly from DOS, your routine will not be preceded by a program segment prefix. You should prevent CHASM from leaving room for a PSP by putting an ORG 0 pseudo-op at the beginning of your routine. If you don't include the ORG, memory references will not be assembled correctly. Example: ORG 0 ;no psp SUBR PROC FAR ;far procedure ... ;body of subroutine RET ENDP CHASM supports two methods for getting assembled routines into BASIC programs. The methods differ in whether the routine is included in the BASIC program file, or in a separate file. To include the code within a BASIC program file, run it through the utility COM2DATA, supplied on your CHASM disk. COM2DATA reads in a .COM format file, and produces a file of equivalent DATA statements which can be merged into a BASIC program. Your program can then READ the data and POKE it into memory. An example program to do this is given on page C-6 of the BASIC manual. An alternative approach would be to store the routine in a string variable, which could later be located with the VARPTR function. If you would prefer to keep your machine language subroutine in a separate file, include a BSAVE pseudo-op somewhere within your assembly language source code. CHASM will build a header on the object code produced, which will mimic that built by BASIC's BSAVE command. The resulting file may be BLOADed by BASIC anywhere in memory. 30 You transfer control to your routine with either the USR function, or the CALL statement. Syntax for these statements can be found in the BASIC manual. 31 XIV. Notes for Those Upgrading to This Version of Chasm. Chasm is not yet cast in stone - improvements and corrections are made fairly frequently, based on both my own experience in using the program, and the comments of outside users. This section summarizes the changes which have been made since version 1.2 was released. Version Notes 2.10 Equated symbols are now allowed in the DB operand list. The status line now gives more information, and error messages are optionally accompanied by a warning 'beep'. 2.09 To resolve a syntactic ambiguity, the first digit of hexadecimal constants must now be in the range 0-9. A leading zero is now permitted on four digit hex constants, to allow fufilling this condition. Note that this change could invalidate programs written under earlier versions of CHASM. 2.08 Configuration process expanded. CHASM now skips over perforations on printed listings. EJECT pseudo-op added. 2.07 Oops. Configuration file now works as advertised. 2.06 CHASM now supports reverse long jumps. The admonition in earlier versions against reverse long jumps was an error. 2.05 A compiled version is now available as a free upgrade to contributors of $20 or more. A new pseudo-op, BSAVE, was added for creation of BLOADable files for use with BASIC. CHASM now supports use of a configuration file to set up a printer. Finally, CHASM.DAT was compacted for faster loading. 2.04 TABs are now expanded and replaced with blanks, for compatibility with IBM text editors. 32 2.03 Two bugs corrected. The first bug involved incorrect assembly of indirect memory references which used a displacement in the range 128-255. The second bug caused a program crash if a hex number longer than 4 digits was in found in the input file. This situation now causes an error message to be printed, and assembly continues. 2.01 The utility COM2DATA was added to the CHASM distribution package as an aid to including machine language subroutines in BASIC programs. 2.00 Corrected a bug in the DS and DB pseudo-ops which caused the last label in a program to be redefined as a memory location, whether it was intended to be one or not. Also, the TAB character was added as a new delimiter, and a 20 page introductory primer was added to the CHASM distribution package. 1.9 The short jump is now represented with mnemonic JMPS, for compatibility with DEBUG version 1.1. Note that this could invalidate programs written under earlier versions of CHASM, which used mnemonic JP for this instruction. 1.8 The operand type "character" was added as a new way to represent immediate data. A single character enclosed in single quotes (') is equivalent to the numeric value of the ASCII code for the character. See section V-1-E of this document for details. 1.7 The DS operator now works for blocks larger than 255 bytes. To keep the listing under control, object code generated by DS is no longer shown on the listing. Also, the OFFSET function now works properly in the displacement field of an indirect memory reference. 1.6 A revision of this document. Some sections were improved slightly, and in response to user requests, a section on execution of assembled programs was added. 33 1.5 Corrected an error which generated the message "Data too Long" if the value FFH was used as 8 bit immediate data. 1.4 User interface improved. Chasm now traps some common input errors such as misspelling a file name, or forgetting to turn on your printer, giving you a second chance rather than just crashing. 1.3 A speed enhancement. Version 1.3 benchmarks about 5 times faster than version 1.2. As can be seen, revisions are made on a regular basis. It is my policy that after once contributing, users are entitled to all future versions free of charge, with no further donation expected. Contributing users are encouraged to send a disk and return mailer from time to time, to receive the most current version. Major improvements will be announced in IPCO INFO and Programmer's Journal. 34 XV. Miscellaneous and A Word From Our Sponsor... A. System Requirements: CHASM requires an 80 column display and at least one disk drive. Under DOS 1.1, 64K of memory is required. DOS 2.0 users should have a minimum of 96K. B. Programming Notes: 1. CHASM is written in BASIC. If you have the compiled version, the source code is available free of charge - just send a disk and stamped return mailer. The program is heavily commented and very modular, so that if you'd like to see how an assembler works, you should read the source listing. CHASM's modular structure makes modification relatively easy, if you'd like to customize. I'd like to hear about any improvements people make, for possible inclusion in future releases. 2. If you do try to modify CHASM, there is a quirk you should know about. Most of the initialization code is in a "transient section" which is used once, then deleted to make more room for the symbol table. Lines 50,000 - 65529 are reserved as this transient section. Note Well: If you run CHASM, then save it to disk, you save the program WITHOUT THE INITIALIZATION CODE. The resulting program is non-functional. Be careful! Keep a backup. C. Red Tape and Legal Nonsense: 1. Disclaimer: CHASM is distributed as is, with no guarantee that it will work correctly in all situations. In no event will the Author be liable for any damages, including lost profits, lost savings or other incidental or consequential damages arising out of the use of or inability to use these programs, even if the Author has been advised of the possibility of such damages, or for any claim by any other party. 35 Despite the somewhat imposing statement above, it *is* my intention to fix any bugs which are brought to my attention. Drop me a line if you think you've found a problem. 2. Copyright Information: The entire CHASM distribution package, consisting of the main program, documentation files, and various data and utility files, is copyright (c) 1983 by David Whitman. The author reserves the exclusive right to distribute this package, or any part thereof, for profit. The name "CHASM (tm)", applied to a microcomputer assembler program, is a trade mark of David Whitman. The package consisting of UNCOMPILED program source code and the various subsidiary files may be copied freely by individuals for evaluation purposes. It is expected that those who find the package useful will make a contribution directly to the author of the program. THE AUTHOR RESERVES THE EXCLUSIVE RIGHT TO DISTRIBUTE THIS PROGRAM IN COMPILED FORM. UNAUTHORIZED DUPLICATION OR DISTRIBUTION OF A COMPILED VERSION OF THIS PROGRAM IS PUNISHABLE BY LAW. D. An Offer You Can't Refuse. CHASM is distributed under a modification of the FREEWARE (tm) marketing scheme, developed by Andrew Fluegelman, author of PC-TALK, whose efforts are gratefully acknowledged. Anyone may obtain a free copy of a relatively slow, but full featured, interpreted version of the program by sending a blank, formatted diskette to the author. An addressed, postage-paid return mailer must accompany the disk (no exceptions, please). 36 A copy of the program, with documentation, will be sent by return mail. The program will carry a notice suggesting a contribution to the program's author. Making a contribution is totally voluntary on the part of the user. Regardless of whether a contribution is made, the user is encouraged to share the program with others. Payment for use is discretionary on the part of each subsequent user. The underlying philosophy here is based on three principles: First, that the value and utility of software is best assessed by the user on his/her own system. Only after using a program can one really determine whether it serves personal applications, needs, and tastes. Second, that the creation of independent personal computer software can and should be supported by the computing community. Finally, that copying and networking of programs should be encouraged, rather than restricted. The ease with which software can be distributed outside traditional commercial channels reflects the strength, rather than the weakness, of electronic information. Those who elect to make a contribution become eligible to receive a free upgrade to a compiled version of the program. The compiled version is functionally equivalent to the interpreted version, but executes much faster. An order form for the compiled version is included at the end of this document. The compiled version is copyrighted, and users are requested NOT to make copies other than for their own use. I am strongly opposed to copy protection, and would regret being forced to protect CHASM. Please recognize the amount of time and money which went into producing CHASM, and respect the author's wishes in this matter. David Whitman 2 North Park St. Apartment L Hanover, NH 03755 (603) 643-2115 37 ********COMPILED VERSION ORDER FORM******** Please copy CHASM version 3.0 onto the enclosed formatted disk. I enclose a self-addressed, stamped mailer. I understand that this compiled version of CHASM is copyrighted, and agree not to distribute any unauthorized copies. My diskette is formatted: __ single sided, no DOS __ doubled sided (DOS optional) Disks may be formatted under either DOS 1.1 or 2.0 Please check one: __ I enclose a donation of $20 or more. __ I am a past contributor of $20 or more. I heard about CHASM from: __ PC World __ Programmer's Journal __ Softalk/IBM __ IPCO Info __ Other:_______________________________________ Name: _______________________________________________________ Address: _______________________________________________________ City, State, Zip: ______________________________________________ ================================================================ Send order form, along with disk, return mailer and check to: David Whitman 2 N. Park St. Apartment L Hanover, NH 03755 Your order will be expedited if you mark your envelope: "VERSION 3.0 UPGRADE" ================================================================