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349 mateuszvis 1
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; rmod - resident module of the SvarCOM command interpreter (NASM code)
349 mateuszvis 3
;
580 mateuszvis 4
; Copyright (C) 2021-2022 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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11
CPU 8086
459 mateuszvis 12
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  acts also as a guardval to detect severe stack overflows
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983 mateusz.vi 24
; Buffer used to remember previous command, when SvarCOM calls the buffered
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; input service at INT 21h,AH=0x0A.
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; This buffer is right before the stack, so in case of a stack overflow event
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; (for example because of a "too ambitious" TSR) only this buffer is damaged,
987 mateusz.vi 28
; and can be invalidated without much harm. To detect such damage, SvarCOM's
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; transient part is appending a signature at the end of the buffer.
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INPUTBUF: times 132 db 0 ; 130 bytes for the input buffer + 2 for signature
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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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366 mateuszvis 37
; 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  ; +4Ah
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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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COMSPECBOOT db "@:\COMMAND.COM", 0 ; +4Ch
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529 mateuszvis 45
; exit code of last application
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LEXCODE  db 0    ; +5Bh
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460 mateuszvis 48
; 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   ; +5Ch
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548 mateuszvis 56
; Program to execute, preset by SvarCOM (128 bytes, ASCIIZ)
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EXECPROG: times 128 db 0                                     ; +6Ah
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548 mateuszvis 59
; 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_OUTFIL:    times 128 db 0     ; +16Ah
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REDIR_OUTAPPEND: dw 0               ; +1EAh
576 mateuszvis 63
REDIR_DEL_STDIN: db 0               ; +1ECh  indicates that the stdin file
64
                                    ;        should be deleted (pipes). This
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                                    ;        MUST contain the 1st char of
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                                    ;        REDIR_INFIL!
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537 mateuszvis 68
; 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:            ; +1EDh
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stc
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retf
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576 mateuszvis 77
skipsig:                  ; +1EFh
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349 mateuszvis 79
; 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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537 mateuszvis 86
; 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
90
 
517 mateuszvis 91
; revert stdin/stdout redirections (if any) to their initial state
92
call REVERT_REDIR_IF_ANY
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576 mateuszvis 94
; redirect stdin and/or stdout if required
543 mateuszvis 95
call REDIR_INOUTFILE_IF_REQUIRED
517 mateuszvis 96
 
460 mateuszvis 97
; should I executed command.com or a pre-set application?
98
or [EXECPROG], byte 0
99
jz EXEC_COMMAND_COM
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461 mateuszvis 101
; 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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107
; exec an application preset (by SvarCOM) in the ExecParamRec
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mov ax, 0x4B00         ; DOS 2+ - load & execute program
461 mateuszvis 109
mov dx, EXECPROG       ; DS:DX  - ASCIZ program name (preset at PSP[already)
460 mateuszvis 110
mov bx, EXEC_PARAM_REC ; ES:BX  - parameter block pointer
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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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463 mateuszvis 114
jmp short skipsig      ; enforce valid ds/ss/etc (can be lost after int 21,4b)
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460 mateuszvis 116
EXEC_COMMAND_COM:
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353 mateuszvis 118
; collect the exit code of previous application
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mov ah, 0x4D
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int 0x21
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mov [LEXCODE], al
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460 mateuszvis 123
; 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 130
; 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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133
; 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]
366 mateuszvis 138
mov dx, [COMSPECPTR]
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USEDEFAULTCOMSPEC:
140
 
349 mateuszvis 141
; prepare the exec param block
459 mateuszvis 142
mov ax, [PSP_ENVSEG]
350 mateuszvis 143
mov [EXEC_PARAM_REC], ax
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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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; 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
153
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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157
; if all went well, jump back to start
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jnc skipsig
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366 mateuszvis 160
; restore DS=CS
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mov bx, cs
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mov ds, bx
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349 mateuszvis 164
; update error string so it contains the error number
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add al, '0'
166
mov [ERRLOAD + 4], al
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366 mateuszvis 168
; display error message
349 mateuszvis 169
mov ah, 0x09
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mov dx, ERRLOAD
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int 0x21
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173
; wait for keypress
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mov ah, 0x08
175
int 0x21
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177
; back to program start
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jmp skipsig
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460 mateuszvis 180
; 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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; 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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366 mateuszvis 186
ERRLOAD db "ERR x, FAILED TO LOAD COMMAND.COM", 13, 10, '$'
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188
; variables used to revert stdin/stdout to their initial state
189
OLD_STDOUT dw 0xffff
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OLD_STDIN  dw 0xffff
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192
 
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; ****************************************************************************
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; *** ROUTINES ***************************************************************
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; ****************************************************************************
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520 mateuszvis 197
; ----------------------------------------------------------------------------
517 mateuszvis 198
; revert stdin/stdout redirections (if any) to their initial state
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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
203
je STDOUT_DONE
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; revert the stdout handle (dst in BX already)
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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 old handle (still in bx)
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mov ah, 0x3e
210
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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214
; is stdin redirected?
215
mov bx, [OLD_STDIN]
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cmp bx, 0xffff
217
je STDIN_DONE
547 mateuszvis 218
; revert the stdin handle (dst in BX already)
543 mateuszvis 219
xor cx, cx       ; src handle (0=stdin)
220
mov ah, 0x46     ; redirect a handle
221
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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227
; delete stdin file if required
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cmp [REDIR_DEL_STDIN], byte 0
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je STDIN_DONE
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; revert the original file and delete it
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mov ah, [REDIR_DEL_STDIN]
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mov [REDIR_INFIL], ah
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mov ah, 0x41     ; DOS 2+ - delete file pointed at by DS:DX
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mov dx, REDIR_INFIL
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int 0x21
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mov [REDIR_INFIL], byte 0
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mov [REDIR_DEL_STDIN], byte 0
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STDIN_DONE:
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ret
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; ----------------------------------------------------------------------------
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244
 
520 mateuszvis 245
; ----------------------------------------------------------------------------
517 mateuszvis 246
; redirect stdout if REDIR_OUTFIL points to something
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REDIR_INOUTFILE_IF_REQUIRED:
548 mateuszvis 248
cmp [REDIR_OUTFIL], byte 0
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je NO_STDOUT_REDIR
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mov si, REDIR_OUTFIL   ; si = output file
517 mateuszvis 251
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], byte 0
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jc NO_STDOUT_REDIR     ; TODO: abort with an error message instead
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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
261
jne SKIP_JMPEOF
262
mov bx, ax
263
mov ax, 0x4202         ; jump to position EOF - CX:DX in handle BX
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xor cx, cx
265
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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517 mateuszvis 270
; 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
548 mateuszvis 273
mov bx, 1              ; 1 = stdout
517 mateuszvis 274
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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517 mateuszvis 277
; 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
520 mateuszvis 282
 
517 mateuszvis 283
; close the original file handle, I no longer need it
284
mov ah, 0x3e           ; close a file handle (handle in BX)
519 mateuszvis 285
int 0x21
517 mateuszvis 286
NO_STDOUT_REDIR:
543 mateuszvis 287
 
288
; *** redirect stdin if REDIR_INFIL points to something ***
548 mateuszvis 289
cmp [REDIR_INFIL], byte 0
543 mateuszvis 290
je NO_STDIN_REDIR
548 mateuszvis 291
mov dx, REDIR_INFIL    ; dx:dx = file
543 mateuszvis 292
mov ax, 0x3d00         ; open file for read
293
int 0x21               ; ax=handle on success (CF clear)
548 mateuszvis 294
mov [REDIR_INFIL], byte 0
543 mateuszvis 295
jc NO_STDIN_REDIR      ; TODO: abort with an error message instead
296
 
297
; duplicate current stdin so I can revert it later
298
push ax                ; save my file handle in stack
299
mov ah, 0x45           ; duplicate file handle BX
300
xor bx, bx             ; 0=stdin
301
int 0x21               ; ax=new (duplicated) file handle
302
mov [OLD_STDIN], ax    ; save the old handle in memory
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304
; redirect stdout to my file
305
pop bx                 ; dst handle
306
xor cx, cx             ; src handle (0=stdin)
307
mov ah, 0x46           ; "redirect a handle"
308
int 0x21
309
 
310
; close the original file handle, I no longer need it
311
mov ah, 0x3e           ; close a file handle (handle in BX)
312
int 0x21
313
NO_STDIN_REDIR:
517 mateuszvis 314
ret
520 mateuszvis 315
; ----------------------------------------------------------------------------