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;
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;
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; rmod - resident module of the SvarCOM command interpreter
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; rmod - resident module of the SvarCOM command interpreter
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;
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;
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; Copyright (C) 2021 Mateusz Viste
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; Copyright (C) 2021 Mateusz Viste
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; MIT license
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; MIT license
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;
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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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; 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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; 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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; and respawning COMMAND.COM whenever necessary.
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CPU 8086
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CPU 8086
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org 0x100
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org 0x100
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PSP_ENVSEG equ 0x2C
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PSP_ENVSEG equ 0x2C
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16 
section .text    ; all goes into code segment
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section .text    ; all goes into code segment
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 17 
 
18 
                 ; offset
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                 ; offset
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SIG1 dw 0x1983   ;  +0
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SIG1 dw 0x1983   ;  +0
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SIG2 dw 0x1985   ;  +2
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SIG2 dw 0x1985   ;  +2
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SIG3 dw 0x2017   ;  +4
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SIG3 dw 0x2017   ;  +4
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SIG4 dw 0x2019   ;  +6  this acts also as a guardval to detect stack overflows
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SIG4 dw 0x2019   ;  +6  this acts also as a guardval to detect stack overflows
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; DOS int 21h functions that I use require at least 40 bytes of stack under
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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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; 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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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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; exit code of last application
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; exit code of last application
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LEXCODE  dw 0    ; +4Ah
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LEXCODE  dw 0    ; +4Ah
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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  ; +4Ch
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COMSPECPTR dw 0  ; +4Ch
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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 ; +4Eh
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COMSPECBOOT db "@:\COMMAND.COM", 0 ; +4Eh
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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   ; +5Dh
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EXEC_PARAM_REC db 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0   ; +5Dh
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; Program to execute, preset by SvarCOM (128 bytes, ASCIIZ)  ; +6Bh
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; Program to execute, preset by SvarCOM (128 bytes, ASCIIZ)  ; +6Bh
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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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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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- 
 
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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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REDIR_OUTFIL dw 0xffff    ; +EBh
- 
 
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REDIR_INFIL dw 0xffff     ; +EDh
- 
 
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REDIR_OUTAPPEND dw 0      ; +EFh
- 
 
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skipsig:         ; +EBh
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skipsig:         ; +F1h
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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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- 
 
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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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- 
 
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; redirect stdout if required
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call REDIR_OUTFILE_IF_REQUIRED
- 
 
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; should I executed command.com or a pre-set application?
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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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or [EXECPROG], byte 0
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jz EXEC_COMMAND_COM
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jz EXEC_COMMAND_COM
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; TODO: perhaps I should call the DOS SetPSP function here? But if I do, the
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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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;       int 21h, ah=50h call freezes...
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;mov ah, 0x50           ; DOS 2+ -- Set PSP
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;mov ah, 0x50           ; DOS 2+ -- Set PSP
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;mov bx, cs
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;mov bx, cs
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;int 0x21
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;int 0x21
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; exec an application preset (by SvarCOM) in the ExecParamRec
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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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mov ax, 0x4B00         ; DOS 2+ - load & execute program
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mov dx, EXECPROG       ; DS:DX  - ASCIZ program name (preset at PSP[already)
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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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mov bx, EXEC_PARAM_REC ; ES:BX  - parameter block pointer
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int 0x21
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int 0x21
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mov [cs:EXECPROG], byte 0 ; do not run app again (+DS might have been changed)
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mov [cs:EXECPROG], byte 0 ; do not run app again (+DS might have been changed)
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jmp short skipsig      ; enforce valid ds/ss/etc (can be lost after int 21,4b)
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jmp short skipsig      ; enforce valid ds/ss/etc (can be lost after int 21,4b)
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EXEC_COMMAND_COM:
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EXEC_COMMAND_COM:
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; collect the exit code of previous application
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; collect the exit code of previous application
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mov ah, 0x4D
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mov ah, 0x4D
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int 0x21
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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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xor ah, ah          ; clear out termination status, I only want the exit code
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mov [LEXCODE], ax
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mov [LEXCODE], ax
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; zero out the exec param block (14 bytes)
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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 al, 0              ; byte to write
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mov cx, 14             ; how many times
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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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mov di, EXEC_PARAM_REC ; ES:DI = destination
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cld                    ; stosb must move forward
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cld                    ; stosb must move forward
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rep stosb              ; repeat cx times
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rep stosb              ; repeat cx times
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; preset the default COMSPEC pointer to ES:DX (ES is already set to DS)
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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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mov dx, COMSPECBOOT
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; do I have a valid COMSPEC?
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; do I have a valid COMSPEC?
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or [COMSPECPTR], word 0
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or [COMSPECPTR], word 0
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jz USEDEFAULTCOMSPEC
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jz USEDEFAULTCOMSPEC
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; set ES:DX to actual COMSPEC (in env segment)
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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 es, [PSP_ENVSEG]
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mov dx, [COMSPECPTR]
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mov dx, [COMSPECPTR]
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USEDEFAULTCOMSPEC:
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USEDEFAULTCOMSPEC:
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; prepare the exec param block
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; prepare the exec param block
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mov ax, [PSP_ENVSEG]
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mov ax, [PSP_ENVSEG]
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mov [EXEC_PARAM_REC], ax
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mov [EXEC_PARAM_REC], ax
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mov [EXEC_PARAM_REC+2], word CMDTAIL
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mov [EXEC_PARAM_REC+2], word CMDTAIL
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mov [EXEC_PARAM_REC+4], cs
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mov [EXEC_PARAM_REC+4], cs
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 122 
 
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; execute command.com
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; execute command.com
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mov ax, 0x4B00         ; DOS 2+ - load & execute program
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mov ax, 0x4B00         ; DOS 2+ - load & execute program
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push es                ;
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push es                ;
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pop ds                 ;
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pop ds                 ;
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;mov dx, COMSPEC       ; DS:DX  - ASCIZ program name (preset already)
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;mov dx, COMSPEC       ; DS:DX  - ASCIZ program name (preset already)
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push cs
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push cs
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pop es
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pop es
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mov bx, EXEC_PARAM_REC ; ES:BX  - parameter block pointer
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mov bx, EXEC_PARAM_REC ; ES:BX  - parameter block pointer
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int 0x21
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int 0x21
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; if all went well, jump back to start
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; if all went well, jump back to start
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jnc skipsig
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jnc skipsig
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; restore DS=CS
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; restore DS=CS
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mov bx, cs
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mov bx, cs
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mov ds, bx
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mov ds, bx
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; update error string so it contains the error number
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; update error string so it contains the error number
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add al, '0'
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add al, '0'
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mov [ERRLOAD + 4], al
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mov [ERRLOAD + 4], al
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; display error message
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; display error message
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mov ah, 0x09
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mov ah, 0x09
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mov dx, ERRLOAD
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mov dx, ERRLOAD
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int 0x21
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int 0x21
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; wait for keypress
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; wait for keypress
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mov ah, 0x08
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mov ah, 0x08
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int 0x21
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int 0x21
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; back to program start
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; back to program start
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jmp skipsig
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jmp skipsig
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; command.com tail arguments, in PSP format: length byte followed by args and
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; command.com tail arguments, in PSP format: length byte followed by args and
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; terminated with \r) - a single 0x0A byte is passed so SvarCOM knows it is
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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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; 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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; this allows multiple copies of SvarCOM to stack upon each other.
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CMDTAIL db 0x01, 0x0A, 0x0D
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CMDTAIL db 0x01, 0x0A, 0x0D
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ERRLOAD db "ERR x, FAILED TO LOAD COMMAND.COM", 13, 10, '$'
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ERRLOAD db "ERR x, FAILED TO LOAD COMMAND.COM", 13, 10, '$'
- 
 
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- 
 
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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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- 
 
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- 
 
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; *** ROUTINES ***************************************************************
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- 
 
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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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; revert the stdout handle (dst in BX alread)
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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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mov [OLD_STDOUT], word 0xffff ; mark stdout as "not redirected"
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STDOUT_DONE:
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ret
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- 
 
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- 
 
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; redirect stdout if REDIR_OUTFIL points to something
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REDIR_OUTFILE_IF_REQUIRED:
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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
- 
 
 201 
jc NO_STDOUT_REDIR     ; TODO: abort with an error message instead
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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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; 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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; 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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NO_STDOUT_REDIR:
- 
 
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ret
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