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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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mateuszvis |
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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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; offset of the COMSPEC variable in the environment block, 0 means "use
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; boot drive". this value is patched by the transient part of COMMAND.COM
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COMSPECPTR dw 0 ; +8Eh
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; fallback COMSPEC string used if no COMPSEC is present in the environment
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; drive. drive is patched by the transient part of COMMAND.COM
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COMSPECBOOT db "@:\COMMAND.COM", 0 ; +90h
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; ECHO status used by COMMAND.COM. 0 = ECHO OFF, 1 = ECHO ON
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CMDECHO db 1 ; +9Fh
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skipsig: ; +A0h
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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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; 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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; preset the default COMSPEC pointer to ES:DX (ES is already set to DS)
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mov dx, COMSPECBOOT
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; do I have a valid COMSPEC?
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or [COMSPECPTR], word 0
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jz USEDEFAULTCOMSPEC
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; set ES:DX to actual COMSPEC
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mov es, [ENVSEG]
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mov dx, [COMSPECPTR]
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USEDEFAULTCOMSPEC:
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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 [EXEC_PARAM_REC+2], dx
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mov [EXEC_PARAM_REC+4], es
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; execute command.com
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mov ax, 0x4B00 ; DOS 2+ - load & execute program
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push es ;
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pop ds ;
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;mov dx, COMSPEC ; DS:DX - ASCIZ program name (preset already)
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push cs
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pop es
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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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; restore DS=CS
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mov bx, cs
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mov ds, bx
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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
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mov ah, 0x09
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mov dx, ERRLOAD
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int 0x21
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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", 13, 10, '$'
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; DOS int 21h functions that I use require at least 32 bytes of stack, here I
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; allocate 64 bytes to be sure
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STACKBUF db "0123456789ABCDEF0123456789ABCDEF0123456789ABCDEF0123456789ABCDEF"
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STACKPTR db "xx"
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