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CHAPTER 7.   DOS INTERFACE




Forth is a language with an operating system packed in.  It provides a
complete programming environment so that a user can write, test and debug
substantial programs completely inside of this environment.  When Chuck
Moore started developing Forth, the operating systems were primitive, and
often got in the way, preventing the user from exploiting the full
capability of the hardware underneath the operating system.  His
solution, adopted by most earlier Forth implementations, was to eliminate
the operating system completely and incorporate all essential functions
of the operating system into Forth.  BLOCK was invented to access mass
storage directly, and it allows Forth to use different magnetic storage
media very efficiently and quite transparently to the user.

However, using blocks imposes many restrictions on the style of
programming in Forth.  The most apparent restriction is on the editor,
which handles text in fixed 1024 byte chunks.  It also makes Forth
incompatible with other programming environments.

In the good old days of minicomputers and mainframes, compatibility was
among the least of concerns because there were so many of them.  None of
them were compatible in hardware or in software.  In the confusion, Forth
was able to offer an universal solution, and was ported easily to every
machine in sight.  It is very sad that these bygone days are no more.  In
the field of microcomputers and personal computers, Big Blue and
Not-So-Big Red have established certain common practices, if not
standards. Whether we like them or not, DOS and MAC prevail as the
environments we are condemned to live in for the next few years.  After
that, we will all be sentenced for life under UNIX if we can ever escape
from C.

It is thus very important that Forth should take advantage of these
less-than-optimized operating systems in areas where convenience and
compatibility are useful and cost effective.  Forth can be then further
refined to arrive at the optimized solutions which demand the best of
performance and code compacting.  F-PC tries to exploit the DOS
environment to its practical limit in adopting the sequential file
structure for mass storage and providing a smooth and convenient path to
all the utility in DOS and OS2.  All DOS and BIOS calls are readily
accessible from F-PC.  You can even spawn a DOS shell within F-PC to use
DOS utilities.


7.1.   SYSTEM INTERRUPTS AND BIOS CALLS


BIOS (Basic Input Output System) is a set of subroutines stored in the
Read-Only Memory (ROM) in the PC.  These subroutines provide the most
elementary services for the operating system to access the resources in
PC, such as the keyboard, the display, the disk drives, and the printer.

To invoke any of these service subroutines, a software interrupt
instruction INT must be executed at the machine level.  This interrupt
instruction transfers the control to the proper service routine via an
interrupt vector table in the memory area 0:0 to 0:3FFH.  Parameters are
sent to the service routine in one or more registers in the CPU.
Results, if any, are returned in the registers or as flags.  For
available BIOS services and their functional specifications, one must
consult the IBM Hardware Reference Manual for the specific model or any
reasonably good book on MS-DOS. See some references in the Preface to
this Manual.

F-PC itself does not provide high level tools to do software interrupts.
However, it does use BIOS service to handle keyboard input, screen and
printer output.  Since most of these words were coded in assembly, they
invoke INT instructions directly.  For example, to receive a character
from the keyboard through BIOS:

        CODE BIOSKEY    ( -- char )
                BEGIN
                        MOV AH, # 0
                        INT 16
                        CMP AX, # 0
                0<> UNTIL
                MOV BIOSKEYVAL AX
                1PUSH
                END-CODE

Other important software interrupts are:

        INT 5           printer
        INT 6           video display
        INT 19          diskette drive
        INT 33          DOS services

If you intend to use any of the software interrupts, you can find lots of
good examples in the source files of F-PC.  Use them as models and modify
them to suit your own need.


7.2.    DOS SERVICE CALLS


INT 33 invokes the DOS services which cover a wide range of very
interesting functions which are not in the BIOS ROM, but loaded into the
RAM memory when DOS is booted.  F-PC uses the DOS services extensively
and a set of words is defined to facilitate their uses.

BDOS                    ( data function -- n )

This service emulates the CP/M BDOS call protocol, which requires a
function number and some data.  The number returned on the stack is zero
always, so that it is compatible with the BDOS function implemented in
F83.

When a service call requires more input and/or output parameters, F-PC
gives you the following choices:

BDOS2                   ( CX DX AX -- CX DX AX )

Pop the top three items on the data stack and then do an INT 33.  The
contents of CX, DX, and AX, at the completion of the DOS service is
returned on the stack.  Most status information is returned in these
registers by DOS.  You have to consult a DOS manual for detailed
specifications on registers.

OS2                     ( CX DX AX -- CX DX AX )

OS2 is identical to BDOS2.  Its sole purpose is to please people in the
three piece suits.  The last item from AX register is masked to an 8 bit
number.

XFDOS                   ( DX CX BX AX ES DS -- CX BX AX Carry )

XFDOS is needed to allow DOS services to access memory above the 64K byte
Code/Data Segment.

DOS 33 Interrupt Services provides more than 30 different types of
services.  It is not possible to cover them here even to a very small
extent.  If you are interested, please consult the MS-DOS systems manual
or books.  In F-PC, the file handles, time/date, directory path, some
keyboard and screen functions are all done through this interrupt
service.

  7.3. THE DOS SHELL


The interface from F-PC to DOS occurs at several levels.  In this section
we will discuss the interface to the DOS commands as opposed to the DOS
system calls.  DOS is a language, with its own syntax rules and
functions.  By providing a bridge to DOS, F-PC encompasses two powerful
languages at a very small cost.

It is convenient to be able to issue DOS commands from within F-PC.  This
avoids having to leave F-PC to do the normal housekeeping works that are
regular occurrences in a DOS based computer. In line with this, F-PC
implements several commands:

$SYS                            ( counted-string -- f1 )

Pass the counted string to COMMAND.COM as a command line.  A shell is
spawned in the process with the "/c" parameter included so that
COMMAND.COM terminates on completion of the command line.  If a NULL
string is passed then the DOS shell will be spawned for you to enter one
or several command lines.  You can then return to Forth by typing EXIT
<return>.

SYS  <commands> (  -- )

Accept a command line following SYS and executes the commands as a DOS
command line.

`  <commands>   ( --  )

Pronounced 'back tick'.  A pseudonym for SYS.

These words allow performing almost any DOS command line operation you
would want to do. To make things even more convenient, several FORTH
words have been defined which use the $SYS for specific DOS functions.
They are as follows:

                DIR  DEL  CHDIR  CD  COPY  REN  RENAME

                A:   B:   C:   D:

These commands may be used exactly as they are used in DOS. If you press
Control-C, or Control-Break during the execution of any of the above
words, the operation will abort and control will be returned back to
Forth.

SYS or ` can be given without a DOS command.  In this case you return the
COMMAND.COM shell and you will see the familiar C> or equivalent DOS
prompt.  You can execute any DOS commands or other COM and EXE files.
After you are through with DOS, EXIT will return you back to F-PC.  You
can switch very conveniently back and forth among Forth, DOS and any
other application.


7.4.    BATCH COMMANDS


F-PC allows commands to be passed to Forth on the DOS command line in the
following manner:

        C> F-PC  <filename>  <forth words>  <return>

This illustrates how you can start F-PC with a specified file name, and
perform commands on the file.  An example of how this might be used is as
follows:

        C>  F-PC  BANNER  LOAD  <return>

Here we are starting F-PC with the file BANNER.SEQ, and then performing
LOAD to compile the file.  Another way to use command line parameters is:

        C>  F-PC  -  <forth words>  <return>

Here we are starting F-PC, followed by a dash "-" to tell F-PC we are not
opening a file, then telling F-PC to perform the Forth words following
the "-".  Here is an example:

        C>  F-PC  -  REPAIR  DIR  <return>

This example starts up F-PC without a file and tells F-PC we want to
repair the word DIR.  It will locate the word DIR and start up the editor
with the cursor located on the first line of the DIR definition.

We can build batch files this way.  As you can see, many types of
commands could be given to F- PC on the command line.  Here are the
contents of the FMETA.BAT:

        F-PC  -  FLOAD  META86

This single line batch file meta-compiles the F-PC kernel file and
creates the KERNEL.COM file. A similar batch file EXTEND.BAT is provided
to extend the the kernel into a full featured F-PC system.  There are a
number of batch files in the F-PC system and they are good examples of
how to call F-PC to perform various tasks in the DOS environment and then
return automatically to DOS.  It is a valuable tool for building
customized applications.

The only caution on batch files is that you must not place commands on
the command line which cause the command line to be interpreted again.
The words HELLO and COLD are such words. They should not be used since
infinite recursion will crash the system.


7.5.   DOS MEMORY MAP OF F-PC

  F-PC was built to handle a class of programming problems that requires
very large memory and cannot be handled well on any normal 64K
implementation of Forth because of space constraints. Careful analysis
showed that in a large Forth system most memory space is consumed by
colon definitions.  This is quite natural because Forth is a virtual
machine.  The virtual machine is built upon a small kernel which has to
be implemented in code definitions.  However, once the basic mechanism is
in place, utilities and applications are constructed in high level using
colon definitions.  Therefore, F-PC struggled to give the user as much
space as he will need to store colon definitions, while reserving only
64K bytes for code, constants, variables, and arrays, and 64K bytes for
heads which may or may not be needed in an application.  F-PC also
includes words with which you can ask the operating system for more
memory when needed and release it back to DOS when you have finished
using it.

F-PC allows the List (colon space) Segment to be much larger than 64K
bytes.  In fact the only real limitation in system size comes from the
fact that there are only 64k available for heads. Calculations indicate
there is room for about 2300 definitions on top of the current system of
about 3000 words.  This calculation is based on an average name length of
5 characters, with an average head length of 12 bytes.  With 5300 total
definitions, less than 30k of code segment would be used for code fields
of colon definitions, leaving the remaining 30+K of Code Segment for
variables and arrays.  Not enough for an infinitely large application
obviously, but probably large enough for most  applications.

F-PC places a segment (ES) and and offset (IP) on the return stack for
each nest operation.  Only the relative segment offset is compiled into
the code field, and the conversion to absolute segment is performed by
NEST.  This makes the code fully relocatable as is required by the DOS
environment.  Some performance could be gained by using absolute segment
addressing, and performing a conversion at save and load time to and from
relative and absolute.  The performance gain would come from having a
simpler NEST, which would not have to perform the relative to absolute
conversion at run-time.

CS, SS, and DS always point to the Code Segment, so that all assembly
language operations work with data in the Code Segment unless a special
operator like @L or !L is used to reach an external memory area.  This
means there is no penalty to be paid for most Forth operations and very
high compatibility to F83 is maintained.

The header of every word contains a view field for locating the source
code, a link field to maintain the vocabulary thread, a name field, and a
code pointer field.  The code pointer field contains a code field address
which points to the executable code of this word in the Code Segment.
The machine code in the Code Segment is executed when this word is
invoked.  For a colon definition, the code starts with a JMP NEST, which
saves the current ES and IP registers, adds the next two bytes to XSEG
and moves them into ES and clears IP to zero.  When the NEST routine is
executed to completion, NEXT obtains the next instruction in the List
Segment pointed to by ES register and starts processing the address list
in this colon definition.

In a code definition, the code pointer field in the header points to its
machine code in the Code Segment.  For other types of words, like
CONSTANTs, VARIABLEs, and other executable structures, the first
instruction in the code field is always a CALL to the appropriate inner
interpreter which carries out the task assigned to the specific type of
word.

This manual is not the proper place to discuss the inner mechanism of
F-PC.  If the above discussion seems to be too abstract, please don't
worry.  F-PC behaves just like any other Forth system.  You can be very
productive without any knowledge about how or why the Forth virtual
machine works.  However, it is very comforting to know where things are
stored, just in case you need to find them.



The memory segments in F-PC
















































Data structures in a colon definition











































7.6.    LONG MEMORY WORD SET


A complete set of words is defined in F-PC to let the user accessing the
DOS memory very conveniently.  The standard Forth memory accessing words
are retained for addressing the Code/Data Segment in the memory.  Generic
extended memory words are appended (or prepended) with the letter L, and
they require address specifications in segment:offset pairs. Words
addressing the List Segment have X prefix and words addressing the Head
Segment have Y prefix.

Generic Long Memory Words are those which require an explicit segment
address with a 16 bit address offset.  They are the basic word set from
which other segment specific memory words are derived.  This word set
includes the following words:

 @L      ( seg addr -- n )       Fetch 16 bit integer.
 !L      ( n seg addr -- )       Store 16 bit integer.
 C@L     ( seg addr -- b )       Fetch a byte.
 C!L     ( b seg addr -- )       Store a byte.
 CMOVEL  ( seg addr seg' addr' n -- ) Move n bytes from seg:addr to seg':addr'.
 CMOVEL> ( seg addr seg' addr' n -- ) Move n bytes in reversed order.
 LFILL   ( seg addr n b -- )     Fill n bytes from seg:addr with b.
 LDUMP   ( seg addr n -- )       Dump n bytes from seg:addr.
 Seg is stored into DUMPSEG.

Following are words addressing the List Segment:

 X,      ( n -- )         Compile integer to top of the list dictionary.
 XC,     ( b -- )         Compile byte to the list dictionary.
 XDUMP   ( rel-seg n -- ) Dump n bytes in the list segment starting at XSEG.
 XDP     ( -- addr )      Offset to top of list dictionary.
 XDPSEG  ( -- seg )       Segment to top of list dictionary.
 XHERE   ( -- seg addr )  Seg:addr to top of list dictionary.
 XSEG    ( -- seg )       Segment base of List Space.

The corresponding words addressing into the Head Segment are:

 Y@      ( addr -- n )    Fetch integer from head segment.
 Y!      ( n addr -- )    Store integer to head segment.
 YC@     ( addr -- b )    Fetch byte.
 YC!     ( b addr -- )    Store byte.
 Y,      ( n -- )         Compile integer to head dictionary.
 YCSET   ( b addr -- )    Set bits in a byte.
 YDUMP   ( addr n -- )    Dump n bytes from addr in the head segment.
 YDP     ( -- addr )      Pointer to top of head dictionary.
 YHERE   ( -- addr )      Address of top of head dictionary.
 YSEG    ( -- seg )       Segment base of Head Space.


7.7.    DOS MEMORY ALLOCATION


The following words allow you to request, adjust and discard memory
blocks from DOS for your program to use.

ALLOC   ( size -- seg2 seg flag )

Request size segments of free memory from DOS.  Flag=0 if ok, 8 if not
enough memory.  Seg is the segment base of the allocated memory.  seg2 is
maximum number of segments available that will not cause an error 8,
insufficient memory.

DEALLOC ( seg -- flag )

Release a block of memory to DOS.  Seg must be the same as the segment
base returned by ALLOC as the base of the allocated memory.  Flag=0 if
ok, 9 if seg is not valid.

SETBLOCK        ( seg size --- flag )

Re-adjust the memory block specified by seg to a new size in segments.
SEG is the absolute segment address that was returned by ALLOC. 'Size' is
the size of the array in 16 byte units. Typically you would take the BYTE
size needed and use the word SEGMENTS to convert that to the number of
segments needed.

These words are useful if you need an extra large space to store large
arrays of data.  F-PC in its default configuration uses about 400K bytes
of memory.  The DOS still has about 200K bytes held in reserve, if you
have not loaded any TSR software programs.  This free memory can be
allocated for your use.  With the extended memory words, you can address
any memory whether DOS allows it or not.  However, if you are building a
system which will coexist with other software, it is prudent to be polite
to Ms. DOS and observe her protocol when it is also convenient to do so.
You make her happy.  She will make you happy.