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mateuszvis |
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mateuszvis |
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; rmod - resident module of the SvarCOM command interpreter (NASM code)
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mateuszvis |
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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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mateuszvis |
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org 0x100
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mateuszvis |
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mateuszvis |
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PSP_ENVSEG equ 0x2C
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mateuszvis |
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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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mateuszvis |
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SIG4 dw 0x2019 ; +6 this acts also as a guardval to detect stack overflows
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mateuszvis |
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mateuszvis |
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; DOS int 21h functions that I use require at least 40 bytes of stack under
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; DOS-C (FreeDOS) kernel, so here I reserve 64 bytes juste to be sure
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STACKBUF db "XXX SVARCOM RMOD BY MATEUSZ VISTE XXXXXXXXXXXXXXXXXXXXXXXXXXXX"
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STACKPTR dw 0
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mateuszvis |
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mateuszvis |
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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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mateuszvis |
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COMSPECPTR dw 0 ; +4Ah
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mateuszvis |
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mateuszvis |
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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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mateuszvis |
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COMSPECBOOT db "@:\COMMAND.COM", 0 ; +4Ch
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mateuszvis |
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mateuszvis |
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; exit code of last application
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LEXCODE db 0 ; +5Bh
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mateuszvis |
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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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mateuszvis |
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EXEC_PARAM_REC db 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 ; +5Ch
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mateuszvis |
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mateuszvis |
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; Program to execute, preset by SvarCOM (128 bytes, ASCIIZ) ; +6Ah
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mateuszvis |
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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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mateuszvis |
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mateuszvis |
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; offset within EXECPROG for out and in filenames in case stdin or stdout
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; needs to be redirected (0xffff=no redirection)
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mateuszvis |
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REDIR_OUTFIL dw 0xffff ; +EAh
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REDIR_INFIL dw 0xffff ; +ECh
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REDIR_OUTAPPEND dw 0 ; +EEh
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mateuszvis |
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mateuszvis |
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; CTRL+BREAK (int 23h) handler
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; According to the TechHelp! Manual: "If you want to abort (exit to the parent
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; process), then set the carry flag and return via a FAR RET. This causes DOS
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; to perform normal cleanup and exit to the parent." (otherwise use iret)
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BREAK_HANDLER: ; +F0h
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stc
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retf
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mateuszvis |
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mateuszvis |
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skipsig: ; +F2h
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mateuszvis |
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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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; set up myself as break handler
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mov ax, 0x2523 ; set int vector 23h
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mov dx, BREAK_HANDLER
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int 0x21
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mateuszvis |
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; revert stdin/stdout redirections (if any) to their initial state
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call REVERT_REDIR_IF_ANY
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; redirect stdout if required
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mateuszvis |
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call REDIR_INOUTFILE_IF_REQUIRED
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mateuszvis |
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mateuszvis |
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; 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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mateuszvis |
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; 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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mateuszvis |
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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
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mateuszvis |
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mov dx, EXECPROG ; DS:DX - ASCIZ program name (preset at PSP[already)
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mateuszvis |
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mov bx, EXEC_PARAM_REC ; ES:BX - parameter block pointer
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int 0x21
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mateuszvis |
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mov [cs:EXECPROG], byte 0 ; do not run app again (+DS might have been changed)
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mateuszvis |
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mateuszvis |
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jmp short skipsig ; enforce valid ds/ss/etc (can be lost after int 21,4b)
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mateuszvis |
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EXEC_COMMAND_COM:
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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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mateuszvis |
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mov [LEXCODE], al
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mateuszvis |
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mateuszvis |
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; 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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mateuszvis |
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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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mateuszvis |
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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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mateuszvis |
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mov dx, [COMSPECPTR]
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USEDEFAULTCOMSPEC:
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mateuszvis |
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; prepare the exec param block
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mateuszvis |
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mov ax, [PSP_ENVSEG]
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mateuszvis |
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mov [EXEC_PARAM_REC], ax
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mateuszvis |
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mov [EXEC_PARAM_REC+2], word CMDTAIL
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mateuszvis |
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mov [EXEC_PARAM_REC+4], cs
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mateuszvis |
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; execute command.com
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mov ax, 0x4B00 ; DOS 2+ - load & execute program
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mateuszvis |
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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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mateuszvis |
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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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mateuszvis |
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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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mateuszvis |
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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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mateuszvis |
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; display error message
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mateuszvis |
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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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mateuszvis |
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; command.com tail arguments, in PSP format: length byte followed by args and
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mateuszvis |
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; terminated with \r) - a single 0x0A byte is passed so SvarCOM knows it is
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; called as respawn (as opposed to being invoked as a normal application)
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; this allows multiple copies of SvarCOM to stack upon each other.
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CMDTAIL db 0x01, 0x0A, 0x0D
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mateuszvis |
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mateuszvis |
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ERRLOAD db "ERR x, FAILED TO LOAD COMMAND.COM", 13, 10, '$'
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mateuszvis |
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; variables used to revert stdin/stdout to their initial state
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OLD_STDOUT dw 0xffff
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OLD_STDIN dw 0xffff
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mateuszvis |
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; ****************************************************************************
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mateuszvis |
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; *** ROUTINES ***************************************************************
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mateuszvis |
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; ****************************************************************************
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mateuszvis |
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mateuszvis |
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; ----------------------------------------------------------------------------
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mateuszvis |
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; revert stdin/stdout redirections (if any) to their initial state
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; all memory accesses are CS-prefixes because this code may be called at
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; times when DS is out of whack.
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REVERT_REDIR_IF_ANY:
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; is stdout redirected?
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mov bx, [OLD_STDOUT]
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cmp bx, 0xffff
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je STDOUT_DONE
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mateuszvis |
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; revert the stdout handle (dst in BX already)
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mateuszvis |
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mov cx, 1 ; src handle (1=stdout)
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mov ah, 0x46 ; redirect a handle
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int 0x21
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mateuszvis |
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; close the old handle (still in bx)
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mov ah, 0x3e
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int 0x21
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mateuszvis |
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mov [OLD_STDOUT], word 0xffff ; mark stdout as "not redirected"
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STDOUT_DONE:
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mateuszvis |
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; is stdin redirected?
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mov bx, [OLD_STDIN]
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cmp bx, 0xffff
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je STDIN_DONE
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; revert the stdout handle (dst in BX already)
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xor cx, cx ; src handle (0=stdin)
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mov ah, 0x46 ; redirect a handle
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int 0x21
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; close the old handle (still in bx)
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mov ah, 0x3e
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int 0x21
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mov [OLD_STDIN], word 0xffff ; mark stdin as "not redirected"
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STDIN_DONE:
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mateuszvis |
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ret
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mateuszvis |
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; ----------------------------------------------------------------------------
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mateuszvis |
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mateuszvis |
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; ----------------------------------------------------------------------------
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mateuszvis |
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; redirect stdout if REDIR_OUTFIL points to something
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mateuszvis |
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REDIR_INOUTFILE_IF_REQUIRED:
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mateuszvis |
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mov si, [REDIR_OUTFIL]
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cmp si, 0xffff
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je NO_STDOUT_REDIR
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add si, EXECPROG ; si=output file
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mov ax, 0x6c00 ; Extended Open/Create
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mov bx, 1 ; access mode (0=read, 1=write, 2=r+w)
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xor cx, cx ; file attribs when(if) file is created (0=normal)
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mov dx, [REDIR_OUTAPPEND] ; action if file exist (0x11=open, 0x12=truncate)
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int 0x21 ; ax=handle on success (CF clear)
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mov [REDIR_OUTFIL], word 0xffff
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jc NO_STDOUT_REDIR ; TODO: abort with an error message instead
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mateuszvis |
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; jump to end of file if flag was 0x11 (required for >> redirections)
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cmp [REDIR_OUTAPPEND], word 0x11
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jne SKIP_JMPEOF
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mov bx, ax
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mov ax, 0x4202 ; jump to position EOF - CX:DX in handle BX
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xor cx, cx
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xor dx, dx
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int 0x21
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mov ax, bx ; put my handle back in ax, as expected by later code
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SKIP_JMPEOF:
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mateuszvis |
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; duplicate current stdout so I can revert it later
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push ax ; save my file handle in stack
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mov ah, 0x45 ; duplicate file handle BX
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mov bx, 1 ; 1=stdout
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int 0x21 ; ax=new (duplicated) file handle
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mov [OLD_STDOUT], ax ; save the old handle in memory
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mateuszvis |
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mateuszvis |
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; redirect stdout to my file
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pop bx ; dst handle
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mov cx, 1 ; src handle (1=stdout)
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mov ah, 0x46 ; "redirect a handle"
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int 0x21
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mateuszvis |
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mateuszvis |
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; close the original file handle, I no longer need it
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mov ah, 0x3e ; close a file handle (handle in BX)
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mateuszvis |
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int 0x21
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mateuszvis |
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NO_STDOUT_REDIR:
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mateuszvis |
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; *** redirect stdin if REDIR_INFIL points to something ***
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mov dx, [REDIR_INFIL]
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cmp dx, 0xffff
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je NO_STDIN_REDIR
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add dx, EXECPROG ; ds:dx=file
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mov ax, 0x3d00 ; open file for read
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int 0x21 ; ax=handle on success (CF clear)
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mov [REDIR_INFIL], word 0xffff
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jc NO_STDIN_REDIR ; TODO: abort with an error message instead
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; duplicate current stdin so I can revert it later
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push ax ; save my file handle in stack
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mov ah, 0x45 ; duplicate file handle BX
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xor bx, bx ; 0=stdin
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int 0x21 ; ax=new (duplicated) file handle
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mov [OLD_STDIN], ax ; save the old handle in memory
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; redirect stdout to my file
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pop bx ; dst handle
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xor cx, cx ; src handle (0=stdin)
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mov ah, 0x46 ; "redirect a handle"
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int 0x21
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; close the original file handle, I no longer need it
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mov ah, 0x3e ; close a file handle (handle in BX)
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int 0x21
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NO_STDIN_REDIR:
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mateuszvis |
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ret
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mateuszvis |
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; ----------------------------------------------------------------------------
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