Appendix D

D.1   EXAMPLE ASSEMBLER SHELL PROGRAM FOR MS-DOS INTERFACING

The Microsoft MACRO-86 assembler follows closely the Intel ASM-86 specifications. The operating system interfacing technique is via a straightforward interrupt (INT 21Hex), with the required operational parameter in the AH register. MS-DOS does not corrupt any registers other than the ones used for the sending or receiving of data. An example of the running and exiting program technique, plus the required assembler directives, follows. The program example is for the small memory model; but it will apply equally well to the compact or large memory model. The 8080 memory model is not recommended as it results in poor usage of the potential of the 8086/8088 processor. At link time, this programming example will generate an .EXE file - the header information on this file type will be found in E.1.

Example of MS-DOS/MACRO-86 Assembly Programming

dgroup  group   data
cgroup  group   code

msdos   equ     00021h             ;interrupt to operating system

data    segment public    'data'
;######  insert your data here ######
data    ends

code    segment public    'code'
        assume  CS: cgroup, DS: dgroup

example proc    near               ;origin of code

begin:
        push    ES                 ;save return segment address
        call    run_module         ;run the program
;
; run ends - select close down
;
exit    proc    far                ;close down code
        xor     ax,ax              ;zero for PSP:0
        push    ax                 ;save for far return
        ret                        ;and close down
exit    endp                       ;close down code ends

run_module:
        mov     ax,DATA            ;get the data segment origin
        mov     DS,ax              ; and initialise the segment
;##### insert your code at this point ######
        ret                        ;return to exit module

        example endp
        code    ends
        end

D.2 Example Assembler Shell Program for CP/M-86 Interfacing

The Digital Research ASM-86 assembler does not follow the standard Intel ASM-86 structure - this makes for a more complex task when transferring assembler programs between the CP/M-86 and the MS-DOS operating systems. The operating system interfacing technique is via a straightforward interrupt (INT E0Hex), with the required operational parameter in the CL register. CP/M-86 corrupts all registers, excepting the CS and IP - it is, therefore, recommended that all registers be pushed prior to the INT E0Hex being issued. An example of the running and exiting program technique, plus the required assembly directives, follows. The program example follows that of the MS-DOS MACRO-86 example. At GENCMD time, this programming example will generate a .CMD file - the header information on this file type is shown in the System Guide for CP/M-86.

Example of CP/M-86/ASM-86 Programming

reset   equ     00000h             ;system reset function
cpm     equ     000e0h             ;interrupt to operating system

        cseg

begin:
        call    run_module         ;run the program
;
; run ends - select close down
;
        mov     cl,reset           ;select system reset
        mov     dl,00h             ;select memory recovery
        int     cpm                ;return to operating system
;
run_module:
;##### insert your code at this point ######
        ret                        ;return to exit module

        dseg
;##### insert your data here #####
        end    

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Last revision 29/01/2005