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Interfacing Turbo Assembler with Turbo Prolog Turbo Prolog offers the programmer a wealth of predicates that provide a rich set of high-level functions, from screen window management to B+ tree database management. What Turbo Assembler adds to Turbo Prolog is a low-level programming facility. In this chapter, we'll first take a look at the interface between Turbo Prolog and Turbo Assembler. Then we'll build some simple examples of interfacing assembly routines to Turbo Prolog. Finally, we'll discuss calling Turbo Prolog predicates from assembly code, using Turbo Prolog library calls and passing compound structures. Note: When referring to Turbo Prolog, we mean versions 1.0 and greater. Declaring external predicates Turbo Prolog allows interfacing with other languages through the use of a global predicates declaration. A language specification is appended to the declaration so that Turbo Prolog knows a global predicate is implemented in another language: global predicates add(integer,integer,integer) - (i,i,o),(i,i,i) language asm scanner(string,token) - (i,o) language Pascal Turbo Prolog makes the interfaced language explicit to simplify the problems of activation record and parameter format, calling and returning conventions, segment definition, linking, and initialization. Calling conventions and parameters The 8086 processor family gives the programmer a choice between near and far subroutine calls. Turbo Prolog creates large memory model programs and requires that all calls to and returns from subroutines be far. Turbo Prolog supports a number of calling conventions; C, Pascal, and assembler. When interfacing to a routine written using the C calling convention, the parameters are pushed onto the stack in reverse order and, after returning, the stack pointer is automatically adjusted. When interfacing to other languages, the parameters are pushed in the normal order, and the called function is responsible for removing the parameters from the stack. In many language compilers for the 8086 family, there is a choice between 16-bit and 32-bit pointers, where the 16-bit pointers refer to a default segment. Turbo Prolog always uses 32-bit pointers to access all memory. Turbo Prolog types are implemented in the following way: integer 2 bytes real 8 bytes (IEEE format) char 1 byte (2 bytes when pushed on the stack) string 4-byte dword pointer to a null-terminated string symbol 4-byte dword pointer to a null-terminated string compound 4-byte dword pointer to a record An output parameter is pushed as a 32-bit pointer to a location where the return value must be assigned. For input parameters, the value is pushed directly, and the size of the parameter depends on its type. Naming conventions The same predicate in Turbo Prolog can have several type variants and several flow variants. Each type and flow variant must have its own procedure that's assigned a unique name. This is done by numbering the different procedures with the same predicate name from 0 upward. For example, given the declaration global predicates add(integer,integer,integer)-(i,i,o),(i,i,i) language asm the first variant--with flow pattern (i,i,o)--is named add_0, and the second--with flow pattern (i,i,i)--is named add_1. Turbo Prolog also allows the programmer to declare an explicit name for a global predicate. This is done by succeeding the declaration with an "as public name" part. In the following example, the global predicate pred will be resolved by an identifier name my_pred, not pred_0. global predicates pred(integer,integer)-(i,o) language asm as "my_pred" This method is good when you're naming predicates that have only one flow pattern. If more than one flow pattern exists, you'll have to provide a name for each variant. Using the add predicate as an example, the predicate definition might read global predicates add(integer,integer,integer)-(i,i,o) language asm as "doadd" add(integer,integer,integer)-(i,i,i) language asm as "add_check" The first variant--with flow pattern (i,i,o)--is named doadd, and the second--with flow pattern (i,i,i)--is named add_check. Note that this naming method requires the variants to be declared separately. Writing assembly language predicates Perhaps the simplest predicates to write are those that have only input flow patterns. Suppose you wanted to scroll the contents of the current Turbo Prolog window horizontally. You could create the predicate scroll_left that scrolls a region of the screen one column to the left. In the SCROLLH.PRO example, scroll_left will take four integer arguments and one flow pattern. The Turbo Prolog module SCROLLH.PRO contains a global predicates declaration for the predicate scroll_left. The scroll_left predicate is defined as an assembly predicate. /* SCROLLH.PRO */ global predicates scroll_left(integer,integer,integer,integer) - (i,i,i,i) language asm predicates scrollh clauses scrollh :- makewindow(_,_,_,_,Row,Col,Nrows,Ncols), scroll_left(Row,Col,Nrows,Ncols), readchar(C), char_int(C,CI), not(CI = 27), scrollh. goal makewindow(1,7,7," A SCROLLING MESSAGE ",10,20,4,60), write("This message will scroll across the window"),nl, write("Look at it go!"), readchar(_), scrollh, readchar(_). The following assembly language source is the implementation of the scroll_left predicate. Notice that the name given to the predicate is SCROLL_LEFT_0. This name conforms (as it must) to the naming conventions discussed earlier in this chapter. ; SCROL.ASM ; name scrol ; ; scroll_left(integer,integer,integer,integer) - ; (i,i,i,i) language asm ; SCROL_TEXT SEGMENT BYTE PUBLIC 'CODE' ASSUME cs:SCROL_TEXT PUBLIC SCROLL_LEFT_0 SCROLL_LEFT_0 PROC FAR ; ; parameters arg NCOLS:WORD, NROWS:WORD, COL:WORD, ROW:WORD = ARGLEN ; ; local variable local SSEG :WORD = LSIZE push bp mov bp,sp sub sp,lsize ;room for local variables push si push di mov SSEG, 0B800h sub NCOLS, 3 ;NCOLS = NCOLS - 3 mov ax, ROW ;DEST = ROW*160 + (COL+1)*2 mov dx,160 mul dx mov dx, COL inc dx ;added shl dx,1 add dx,ax push ds push es mov bx , NROWS ;loop NROWS times using BX as counter dec bx ;NROWS = NROWS - 2 dec bx Top: cmp bx ,0 je Done add dx, 160 ;dest = dest + 160 mov ax,NCOLs ;lastchar = dest + nc*2 shl ax,1 add ax,dx push ax ;push lastchar offset on stack mov ax,SSEG ;load screen segment into ES, DS mov es,ax mov ds,ax mov di,dx ;set up SI and DI for movs mov si,di ;source is 2 bytes above DEST add si,2 mov ax,[di] ;save the char in col 0 in AX mov cx,NCOLS ;mov NCOLS words cld rep movsw pop di ;pop lastchar offset to DI mov [di],ax ;put char in AX to last column dec bx jmp TOP Done:pop es pop ds pop di pop si mov sp,bp pop bp ret ARGLEN SCROLL_LEFT_0 ENDP SCROL_TEXT ENDS END To create an executable file from SCROLLH.PRO and SCROL.ASM, first compile the Turbo Prolog file to an .OBJ file using Turbo Prolog. (When Turbo Prolog compiles a module, it creates an .OBJ file and a .SYM file.) Then assemble SCROL.ASM to an .OBJ file with Turbo Assembler, and link the modules together with the following TLINK command line: TLINK init scrollh scrol scrollh.sym,scroll,,prolog The resultant executable file will be named SCROLL.EXE. Implementing the double predicate Suppose an assembly language routine is to be called into operation via the clause double(MyInVar,MyOutVar) with MyInVar bound to an integer value before the call so that, after the call, MyOutVar is bound to twice that value. The activation record placed on top of the stack when double is activated will take the form shown in Figure 2.1. Figure 2.1 Activation record for double The following assembly language function implements the double predicate: ; ; MYASM.ASM ; A_PROG SEGMENT BYTE ASSUME cs:a_prog PUBLIC double_0 double_0 PROC FAR push bp mov bp,sp mov ax, [bp]+6 ;get the value to which ; MyInVar is bound add ax,ax ;double that value lds si,DWORD PTR [bp]+10 mov [si],ax ;store the value to which ; MyOutVar is to be bound in ; the appropriate address pop bp mov sp,bp ret 6 double_0 ENDP A_PROG ENDS The Turbo Prolog program containing the call to double must contain the following global predicates declaration: global predicates double(integer,integer) - (i,o) language asm Otherwise, the Turbo Prolog program is no different from any other program. The following program uses this double: /* MYPROLOG.PRO */ global predicates double(integer,integer) - (i,o) language asm goal write("Enter an integer "), readint(I), double(I,Y), write(I," doubled is ",Y). If this assembly language program module is assembled into the file MYASM.OBJ, and the calling Turbo Prolog object module is MYPROLOG.OBJ, the two can be linked via this command line TLINK init myprolog myasm myprolog.sym,double,,prolog and produce an executable, stand-alone program in the file DOUBLE.EXE (using the Turbo Prolog library in PROLOG.LIB). It is important that MYPROLOG.SYM appear as the last file name before the first comma in the TLINK command. In general, the format of an activation record will depend on the number of parameters in the calling Turbo Prolog predicate and the domain types corresponding to those parameters. For instance, if you wanted to define add(Val1,Val2,Sum) with Val1, Val2, and Sum belonging to integer domains, the activation record would take the form shown in Figure 2.2. Figure 2.2 Activation record for add Notice that each parameter occupies a corresponding number of bytes. For output parameters, the size is always 4 bytes (used for segment address and offset). For input parameters, the size is determined by the value actually pushed onto the stack, so it is dependent on the corresponding domain. Val1 and Val2, belonging to an integer domain and being used with an (i) flow pattern, both occupy 2 bytes; Sum, being used with an (o) flow pattern, occupies 4 bytes. Note also that, within the Turbo Prolog compiler, a call to an external predicate takes the form mov ax,SEGMENT data mov ds,ax call FAR PTR external_predicate_implementation so the data segment that's addressed while a procedure for an external predicate is being executed is the segment called DATA. Implementing predicates with multiple flow patterns When implementing predicates with multiple flow patterns, you must be careful that the assembly language functions adhere to the Turbo Prolog naming convention. For example, suppose you want to implement the predicate add, which has multiple flow patterns. add will find the missing value in the equation X + Y = Z when any two of the three arguments are bound at the time of the call to add. The following Turbo Prolog program, ADDPRO.PRO, declares the global assembly language predicate add. Notice that add has three flow patterns, (i,i,o), (i,o,i), and (o,i,i). /* ADDPRO.PRO */ global predicates add(integer,integer,integer) - (i,i,o),(i,o,i),(o,i,i) language asm goal add(2,3,X), write("2 + 3 = ",X),nl, add(2,Y,5), write("5 - 2 = ",Y),nl, add(Z,3,5), write("5 - 3 = ",Z),nl. The following assembly language program, ADD.ASM, contains the code to implement add. ADD_0 corresponds to the (i,i,o) flow pattern, ADD_1 corresponds to (i,o,i), and ADD_2 to (o,i,i). name add ADD_TEXT SEGMENT BYTE PUBLIC 'CODE' ASSUME cs:ADD_TEXT PUBLIC ADD_0 ;(i,i,o) flow pattern ADD_0 PROC FAR arg Z:DWORD, Y:WORD, X:WORD = ARGLEN1 push bp mov bp,sp mov ax,X add ax,Y les bx,Z mov WORD PTR es:[bx],ax pop bp ret ARGLEN1 ADD_0 ENDP PUBLIC ADD_1 ;(i,o,i) flow pattern ADD_1 PROC FAR arg Z:WORD, Y:DWORD, X:WORD = ARGLEN2 push bp mov bp,sp mov ax, Z sub ax, X les bx, Y mov WORD PTR es:[bx],ax pop bp ret ARGLEN2 ADD_1 ENDP PUBLIC ADD_2 ;(o,i,i) flow pattern ADD_2 PROC FAR arg Z:WORD, Y:WORD, X:DWORD = ARGLEN3 push bp mov bp,sp mov ax, Z sub ax, Y les bx, X mov WORD PTR es:[bx],ax pop bp ret ARGLEN3 ADD_2 ENDP ADD_TEXT ENDS END After ADDPRO.PRO and ADD.ASM have been translated to .OBJ files, you can create an .EXE file with the following command line: TLINK init addpro add addpro.sym,addpro,,prolog Calling Turbo Prolog predicates from assembly functions Now that we've discussed calling assembly language functions from Turbo Prolog, we'll cover it in reverse--calling Turbo Prolog predicates from assembly language. When a predicate is declared as a global predicate, the predicate's variants become global functions that can be called by any other module. The naming and calling conventions are the same as for predicates defined in assembly language. The following Turbo Prolog module defines two global predicates: popmessage and from_asm. popmessage is declared as a C language predicate and from_asm is declared as an assembly language predicate. To build SHOWMESS, compile SHOWMESS.PRO to .OBJ from the Turbo Prolog development environment. Then compile FROM_ASM.ASM with tasm from_asm and link with TLINK init showmess from_asm showmess.sym,showmess,,prolog Here's SHOWMESS: /* SHOWMESS.PRO */ global predicates popmessage(string) - (i) language c /* predicate called from assembly language procedure */ from_asm - language asm /* assembly language procedure */ clauses popmessage(S) :- /* can be called as a c function named popmessage_0 */ str_len(S,L), LL = L + 4, makewindow(13,7,7,"",10,10,3,LL), write(S), readchar(_), removewindow. goal from_asm. /* external */ The following assembly code implements from_asm and issues a call to popmessage: EXTRN PopMessage_0:FAR DGROUP GROUP _DATA ASSUME cs:SENDMESS_TEXT,ds:DGROUP _DATA SEGMENT WORD PUBLIC 'DATA' mess1 DB "Report: Condition Red",0 _DATA ENDS SENDMESS_TEXT SEGMENT BYTE PUBLIC 'CODE' PUBLIC FROM_ASM_0 FROM_ASM_0 PROC FAR push ds mov ax,OFFSET DGROUP:mess1 push ax call FAR PTR PopMessage_0 pop cx pop cx ret FROM_ASM_0 ENDP SENDMESS_TEXT ENDS END A program follows, using high-level assembly extensions to build the same executable program. To build it, compile SHOWNEW.PRO to .OBJ from the Turbo Prolog development environment, then compile FROM_ASM.ASM with tasm /jmasm51 /jquirks from_new and link with TLINK init shownew from_new shownew.sym,show2,,prolog Here's SHOWNEW: /* SHOWNEW.PRO */ global predicates popmessage(string) - (i) language c /* predicate called from assembly language procedure */ from_asm - language c as "_from_asm" /* define public name of assembly language procedure */ clauses popmessage(S) :- str_len(S,L), LL=L+4, makewindow(13,7,7,"window",10,10,3,LL), write(S), readchar(_), removewindow. goal from_asm. /* call assembly language procedure */ The following assembly code implements from_asm and issues a call to popmessage (like in the preceding example). ; FROM_NEW.ASM EXTRN PopMessage_0:far .MODEL large,c .CODE FROM_ASM PROC push ds mov ax, OFFSET DGROUP:mess1 push ax call FAR PTR PopMessage_0 pop cx pop cx ret FROM_ASM ENDP .DATA mess1 DB "Report: Condition Red",0 END Lists and functors In this section, we'll discuss the method used to pass lists and functors to assembly language predicates. As mentioned previously, compound Turbo Prolog objects are not passed directly; instead, Turbo Prolog passes a 4-byte pointer to a structure. The record structure used for both lists and functors is simple and straightforward. Suppose you had the following Turbo Prolog domains: domains ilist = integer* ifunc = int(integer) The corresponding list-node structure for the ilist domain would be as follows: STRUC ILIST NodeType DB ? Value DW ? NextNode DD ? ENDS As you can see from this structure, a list node has three parts: the node type (a byte) the node value (depends on the type) the pointer to the next node (4 bytes) The node type can contain two meaningful values: Value 1 means that the node is a list node, while Value 2 means that the node is an end-of-list node. (An end-of-list node contains no other meaningful information.) The node value can be any Turbo Prolog domain. The corresponding structure for the ifunc functor would be as follows: STRUC IFUNC FuncType DB ? Value DW ? ENDS A functor structure has two parts: the functor type and the functor record. The functor type is an integer associated with the position of the functor variant in the list of alternates. The first alternate is type 1, the second is type 2, and so on. In the following Turbo Prolog and Turbo Assembler modules, we've implemented a predicate that returns a functor to Turbo Prolog: Here's the Turbo Prolog module: /* FUNC.PRO */ domains ifunc = int(integer) global predicates makeifunc(integer,ifunc) - (i,o) language c goal makeifunc(4,H), write(H). And this is the Turbo Assembler module: ; ; IFUNC.ASM ; EXTRN _alloc_gstack:FAR ;_alloc_gstack returns ; pointer to memory block STRUC IFUNC FuncType DB ? Value DW ? ENDS IFUNC_TEXT SEGMENT WORD PUBLIC 'CODE' ASSUME cs:IFUNC_TEXT PUBLIC Makeifunc_0 Makeifunc_0 PROC FAR arg __inval:WORD, __outp:DWORD push bp mov bp,sp mov ax,3 ;allocate 3 bytes push ax call FAR PTR _alloc_gstack pop cx les bx,__outp mov [WORD PTR es:bx+2],dx mov [WORD PTR es:bx],ax mov ax,__inval ;; les bx,__outp les bx,[DWORD PTR es:bx] mov [(IFUNC PTR es:bx).VALUE],ax ;value = __inval mov [(IFUNC PTR es:bx).FUNCTYPE],1 ;type = 1 pop bp ret Makeifunc_0 ENDP IFUNC_TEXT ENDS END This example used only one functor type for ifunc. If you were to declare another functor, like so: myfunc = int(integer); char(char); r(real); d2(integer,real) the structure would become somewhat more complicated. The structure would still have two parts, but the second part would be a union of the data structures required to define all the variants for myfunc. The following structure is a possible implementation of myfunc in Turbo Assembler: STRUC MyFunc FuncType DB ? UNION STRUC _int DW ? ENDS STRUC _char DB ? ENDS STRUC _real DQ ? ENDS STRUC v1 DW ? v2 DQ ? ENDS ENDS ENDS The types associated with functor alternates would be int(integer) 1 char(char) 2 r(real) 3 d2(integer,real) 4 To help put lists and functors into focus, take a look at the earlier domains declaration of ilist. Why are the valid node types 1 and 2? Because Turbo Prolog treats ilist as a structure that could have been just as easily declared as ilist = listnode(integer,listnode); end_of_list. Keep in mind that when you pass compound objects, you pass a pointer to a structure. More specifically: An input flow pattern list or functor is passed by reference; an output flow pattern list or functor is passed as a pointer to a reference to a structure. (Turbo Prolog passes the address of a pointer to the returned structure.) All structures returned to Turbo Prolog should use memory allocated with Turbo Prolog's memory allocation functions. (Refer to the Turbo Prolog User's Guide and the Turbo Prolog Reference Guide.)