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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 0x100
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459 mateuszvis 14 
PSP_ENVSEG equ 0x2C
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349 mateuszvis 16 
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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461 mateuszvis 24 
FFU_UNUSED dw 0  ;  +8
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353 mateuszvis 26 
; exit code of last application
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LEXCODE  dw 0    ; +0Ah
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366 mateuszvis 29 
; 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 33 
; 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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; 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   ; +1Dh
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461 mateuszvis 45 
; Program to execute, preset by SvarCOM (128 bytes, ASCIIZ)  ; +2Bh
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EXECPROG dd 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0
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461 mateuszvis 48 
skipsig:         ; +ABh
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349 mateuszvis 50 
; 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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460 mateuszvis 57 
; should I executed command.com or a pre-set application?
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or [EXECPROG], byte 0
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jz EXEC_COMMAND_COM
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461 mateuszvis 61 
; TODO: perhaps I should call the DOS SetPSP function here? But if I do, the
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;       int 21h, ah=50h call freezes...
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;mov ah, 0x50           ; DOS 2+ -- Set PSP
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;mov bx, cs
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;int 0x21
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; exec an application preset (by SvarCOM) in the ExecParamRec
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mov ax, 0x4B00         ; DOS 2+ - load & execute program
461 mateuszvis 69 
mov dx, EXECPROG       ; DS:DX  - ASCIZ program name (preset at PSP[already)
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mov bx, EXEC_PARAM_REC ; ES:BX  - parameter block pointer
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int 0x21
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mov [EXECPROG], byte 0 ; make sure to spawn command.com after app exits
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EXEC_COMMAND_COM:
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353 mateuszvis 76 
; 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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460 mateuszvis 82 
; zero out the exec param block (14 bytes)
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mov al, 0              ; byte to write
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mov cx, 14             ; how many times
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mov di, EXEC_PARAM_REC ; ES:DI = destination
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cld                    ; stosb must move forward
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rep stosb              ; repeat cx times
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366 mateuszvis 89 
; 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 (in env segment)
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mov es, [PSP_ENVSEG]
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mov dx, [COMSPECPTR]
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USEDEFAULTCOMSPEC:
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349 mateuszvis 100 
; prepare the exec param block
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mov ax, [PSP_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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366 mateuszvis 120 
; restore DS=CS
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mov bx, cs
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mov ds, bx
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349 mateuszvis 124 
; 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 128 
; display error message
349 mateuszvis 129 
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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460 mateuszvis 140 
; command.com tail arguments, in PSP format: length byte followed by args and
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; terminated with \r)
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CMDTAIL db 0x00, 0x0D
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366 mateuszvis 144 
ERRLOAD db "ERR x, FAILED TO LOAD COMMAND.COM", 13, 10, '$'
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366 mateuszvis 146 
; 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
349 mateuszvis 148 
STACKBUF db "0123456789ABCDEF0123456789ABCDEF0123456789ABCDEF0123456789ABCDEF"
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STACKPTR db "xx"