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mateuszvis |
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; rmod - resident module of the SvarCOM command interpreter
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;
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; Copyright (C) 2021 Mateusz Viste
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; MIT license
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;
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; this is installed in memory by the transient part of SvarCOM. it has only
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; two jobs: providing a resident buffer for command history, environment, etc
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; and respawning COMMAND.COM whenever necessary.
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CPU 8086
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org 0h ; this is meant to be executed without a PSP
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section .text ; all goes into code segment
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mateuszvis |
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; offset
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SIG1 dw 0x1983 ; +0
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SIG2 dw 0x1985 ; +2
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SIG3 dw 0x2017 ; +4
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SIG4 dw 0x2019 ; +6
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; environment segment - this is updated by SvarCOM at init time
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ENVSEG dw 0 ; +8
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; exit code of last application
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LEXCODE dw 0 ; +0Ah
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; input buffer used for the "previous command" history
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mateuszvis |
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BUF000 db 128, 0 ; +0Ch
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BUF064 db "0123456789ABCDEF0123456789ABCDEF0123456789ABCDEF0123456789ABCDEF"
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BUF128 db "0123456789ABCDEF0123456789ABCDEF0123456789ABCDEF0123456789ABCDEF"
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skipsig: ; +8Eh
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; set up CS=DS=SS and point SP to my private stack buffer
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mov ax, cs
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mov ds, ax
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mov es, ax
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mov ss, ax
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mov sp, STACKPTR
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mateuszvis |
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; collect the exit code of previous application
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mov ah, 0x4D
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int 0x21
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xor ah, ah ; clear out termination status, I only want the exit code
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mov [LEXCODE], ax
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; prepare the exec param block
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mov ax, [ENVSEG]
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mov [EXEC_PARAM_REC], ax
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mov ax, COMSPEC
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mov [EXEC_PARAM_REC+2], ax
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mov [EXEC_PARAM_REC+4], ds
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; execute command.com
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mov ax, 0x4B00 ; DOS 2+ - load & execute program
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mov dx, COMSPEC ; DS:DX - ASCIZ program name TODO: use real COMSPEC...
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mov bx, EXEC_PARAM_REC ; ES:BX - parameter block pointer
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int 0x21
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; if all went well, jump back to start
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jnc skipsig
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; update error string so it contains the error number
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add al, '0'
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mov [ERRLOAD + 4], al
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; display error message (with trailing COMSPEC)
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mov ah, 0x09
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mov dx, ERRLOAD
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mov [COMSPCZ], byte '$' ; patch comspec terminator to be $
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int 0x21
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mov [COMSPCZ], byte 0 ; restore initial (NULL) compsec terminator
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; wait for keypress
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mov ah, 0x08
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int 0x21
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; back to program start
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jmp skipsig
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; ExecParamRec used by INT 21h, AX=4b00 (load and execute program), 14 bytes:
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; offset size content
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; +0 2 segment of environment for child (0 = current)
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; +2 4 address of command line to place at PSP:0080
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; +6 4 address of an FCB to be placed at PSP:005c
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; +0Ah 4 address of an FCB to be placed at PSP:006c
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EXEC_PARAM_REC db 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0
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ERRLOAD db "ERR x, FAILED TO LOAD COMMAND.COM FROM:", 13, 10
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COMSPEC db "C:\SVN\SVARDOS\SVARCOM\COMMAND.COM"
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COMSPCZ db 0
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; FreeDOS int 21h functions that I use require at least 32 bytes of stack,
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; here I allocate 64 bytes to be sure
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STACKBUF db "0123456789ABCDEF0123456789ABCDEF0123456789ABCDEF0123456789ABCDEF"
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STACKPTR db "xx"
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