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349 mateuszvis 1 
;
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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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                 ; 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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350 mateuszvis 22 
; 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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366 mateuszvis 28 
; 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  ; +0Ch
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366 mateuszvis 32 
; 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 ; +0Eh
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450 mateuszvis 36 
skipsig:         ; +1Dh
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349 mateuszvis 38 
; 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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366 mateuszvis 51 
; 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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349 mateuszvis 62 
; 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, CMDTAIL
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mov [EXEC_PARAM_REC+2], ax
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mov [EXEC_PARAM_REC+4], cs
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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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349 mateuszvis 86 
; 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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366 mateuszvis 90 
; 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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442 mateuszvis 102 
; command.com tail arguments, in PSP format (length byte followed by arg)
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CMDTAIL db 0
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349 mateuszvis 105 
; 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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366 mateuszvis 113 
ERRLOAD db "ERR x, FAILED TO LOAD COMMAND.COM", 13, 10, '$'
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366 mateuszvis 115 
; 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"