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Rev 987 Rev 1178
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STACKBUF db "XXX  SVARCOM RMOD BY MATEUSZ VISTE  XXXXXXXXXXXXXXXXXXXXXXXXXXXX"
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STACKBUF db "XXX  SVARCOM RMOD BY MATEUSZ VISTE  XXXXXXXXXXXXXXXXXXXXXXXXXXXX"
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STACKPTR dw 0
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STACKPTR dw 0
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; offset of the COMSPEC variable in the environment block, 0 means "use
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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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; boot drive". this value is patched by the transient part of COMMAND.COM
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COMSPECPTR dw 0  ; +4Ah
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COMSPECPTR dw 0  ; +CEh
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; fallback COMSPEC string used if no COMPSEC is present in the environment
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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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; drive. drive is patched by the transient part of COMMAND.COM
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COMSPECBOOT db "@:\COMMAND.COM", 0 ; +4Ch
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COMSPECBOOT db "@:\COMMAND.COM", 0 ; +D0h
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; exit code of last application
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; exit code of last application
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LEXCODE  db 0    ; +5Bh
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LEXCODE  db 0    ; +DFh
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; ExecParamRec used by INT 21h, AX=4b00 (load and execute program), 14 bytes:
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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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;  offset  size  content
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;     +0     2   segment of environment for child (0 = current)
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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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;     +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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;     +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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;    +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   ; +5Ch
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EXEC_PARAM_REC db 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0   ; +E0h
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; Program to execute, preset by SvarCOM (128 bytes, ASCIIZ)
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; Program to execute, preset by SvarCOM (128 bytes, ASCIIZ)
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EXECPROG: times 128 db 0                                     ; +6Ah
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EXECPROG: times 128 db 0                                     ; +EEh
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; File where stdin and stdout should be redirected (0 = no redirection)
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; File where stdin and stdout should be redirected (0 = no redirection)
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REDIR_INFIL:     times 128 db 0     ; +EAh
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REDIR_INFIL:     times 128 db 0     ; +16Eh
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REDIR_OUTFIL:    times 128 db 0     ; +16Ah
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REDIR_OUTFIL:    times 128 db 0     ; +1EEh
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REDIR_OUTAPPEND: dw 0               ; +1EAh
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REDIR_OUTAPPEND: dw 0               ; +26Eh
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REDIR_DEL_STDIN: db 0               ; +1ECh  indicates that the stdin file
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REDIR_DEL_STDIN: db 0               ; +270h  indicates that the stdin file
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                                    ;        should be deleted (pipes). This
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                                    ;        should be deleted (pipes). This
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                                    ;        MUST contain the 1st char of
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                                    ;        MUST contain the 1st char of
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                                    ;        REDIR_INFIL!
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                                    ;        REDIR_INFIL!
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; CTRL+BREAK (int 23h) handler
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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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; 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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; 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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; to perform normal cleanup and exit to the parent." (otherwise use iret)
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BREAK_HANDLER:            ; +1EDh
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BREAK_HANDLER:            ; +271h
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stc
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stc
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retf
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retf
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- 
 
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; INT 0x2E handler
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INT2E:
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xor ax, ax
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iret
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skipsig:                  ; +1EFh
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skipsig:                  ; +276h
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; set up CS=DS=SS and point SP to my private stack buffer
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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 ax, cs
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mov ds, ax
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mov ds, ax
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mov es, ax
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mov es, ax
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mov ss, ax
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mov ss, ax
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mov sp, STACKPTR
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mov sp, STACKPTR
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; set up myself as break handler
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; set up myself as break handler (int 0x23)
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mov ax, 0x2523  ; set int vector 23h
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mov ax, 0x2523  ; set int vector 23h
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mov dx, BREAK_HANDLER
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mov dx, BREAK_HANDLER
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int 0x21
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int 0x21
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- 
 
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; set up myself as int 0x2E handler ("pass command to shell")
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mov ax, 0x252E
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mov dx, INT2E ; TODO do something meaningful instead of a no-op
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int 0x21
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; revert stdin/stdout redirections (if any) to their initial state
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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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call REVERT_REDIR_IF_ANY
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; redirect stdin and/or stdout if required
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; redirect stdin and/or stdout if required
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call REDIR_INOUTFILE_IF_REQUIRED
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call REDIR_INOUTFILE_IF_REQUIRED