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Rev 1178 Rev 1203
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;
 1 
;
2 
; rmod - resident module of the SvarCOM command interpreter (NASM code)
 2 
; rmod - resident module of the SvarCOM command interpreter (NASM code)
3 
;
 3 
;
4 
; Copyright (C) 2021-2022 Mateusz Viste
 4 
; Copyright (C) 2021-2022 Mateusz Viste
5 
; MIT license
 5 
; MIT license
6 
;
 6 
;
7 
; this is installed in memory by the transient part of SvarCOM. it has only
 7 
; this is installed in memory by the transient part of SvarCOM. it has only
8 
; two jobs: providing a resident buffer for command history, environment, etc
 8 
; two jobs: providing a resident buffer for command history, environment, etc
9 
; and respawning COMMAND.COM whenever necessary.
 9 
; and respawning COMMAND.COM whenever necessary.
10 
 
 10 
 
11 
CPU 8086
 11 
CPU 8086
12 
org 0x100
 12 
org 0x100
13 
 
 13 
 
14 
PSP_ENVSEG equ 0x2C
 14 
PSP_ENVSEG equ 0x2C
15 
 
 15 
 
16 
section .text    ; all goes into code segment
 16 
section .text    ; all goes into code segment
17 
 
 17 
 
18 
                 ; offset
 18 
                 ; offset
19 
SIG1 dw 0x1983   ;  +0
 19 
SIG1 dw 0x1983   ;  +0
20 
SIG2 dw 0x1985   ;  +2
 20 
SIG2 dw 0x1985   ;  +2
21 
SIG3 dw 0x2017   ;  +4
 21 
SIG3 dw 0x2017   ;  +4
22 
SIG4 dw 0x2019   ;  +6  acts also as a guardval to detect severe stack overflows
 22 
SIG4 dw 0x2019   ;  +6  acts also as a guardval to detect severe stack overflows
23 
 
 23 
 
24 
; Buffer used to remember previous command, when SvarCOM calls the buffered
 24 
; Buffer used to remember previous command, when SvarCOM calls the buffered
25 
; input service at INT 21h,AH=0x0A.
 25 
; input service at INT 21h,AH=0x0A.
26 
; This buffer is right before the stack, so in case of a stack overflow event
 26 
; This buffer is right before the stack, so in case of a stack overflow event
27 
; (for example because of a "too ambitious" TSR) only this buffer is damaged,
 27 
; (for example because of a "too ambitious" TSR) only this buffer is damaged,
28 
; and can be invalidated without much harm. To detect such damage, SvarCOM's
 28 
; and can be invalidated without much harm. To detect such damage, SvarCOM's
29 
; transient part is appending a signature at the end of the buffer.
 29 
; transient part is appending a signature at the end of the buffer.
30 
INPUTBUF: times 132 db 0 ; 130 bytes for the input buffer + 2 for signature
 30 
INPUTBUF: times 132 db 0 ; 130 bytes for the input buffer + 2 for signature
31 
 
 31 
 
32 
; DOS int 21h functions that I use require at least 40 bytes of stack under
 32 
; DOS int 21h functions that I use require at least 40 bytes of stack under
33 
; DOS-C (FreeDOS) kernel, so here I reserve 64 bytes juste to be sure
 33 
; DOS-C (FreeDOS) kernel, so here I reserve 64 bytes juste to be sure
34 
STACKBUF db "XXX  SVARCOM RMOD BY MATEUSZ VISTE  XXXXXXXXXXXXXXXXXXXXXXXXXXXX"
 34 
STACKBUF db "XXX  SVARCOM RMOD BY MATEUSZ VISTE  XXXXXXXXXXXXXXXXXXXXXXXXXXXX"
35 
STACKPTR dw 0
 35 
STACKPTR dw 0
36 
 
 36 
 
37 
; offset of the COMSPEC variable in the environment block, 0 means "use
 37 
; offset of the COMSPEC variable in the environment block, 0 means "use
38 
; boot drive". this value is patched by the transient part of COMMAND.COM
 38 
; boot drive". this value is patched by the transient part of COMMAND.COM
39 
COMSPECPTR dw 0  ; +CEh
 39 
COMSPECPTR dw 0  ; +CEh
40 
 
 40 
 
41 
; fallback COMSPEC string used if no COMPSEC is present in the environment
 41 
; fallback COMSPEC string used if no COMPSEC is present in the environment
42 
; drive. drive is patched by the transient part of COMMAND.COM
 42 
; drive. drive is patched by the transient part of COMMAND.COM
43 
COMSPECBOOT db "@:\COMMAND.COM", 0 ; +D0h
 43 
COMSPECBOOT db "@:\COMMAND.COM", 0 ; +D0h
44 
 
 44 
 
45 
; exit code of last application
 45 
; exit code of last application
46 
LEXCODE  db 0    ; +DFh
 46 
LEXCODE  db 0    ; +DFh
47 
 
 47 
 
48 
; ExecParamRec used by INT 21h, AX=4b00 (load and execute program), 14 bytes:
 48 
; ExecParamRec used by INT 21h, AX=4b00 (load and execute program), 14 bytes:
49 
;  offset  size  content
 49 
;  offset  size  content
50 
;     +0     2   segment of environment for child (0 = current)
 50 
;     +0     2   segment of environment for child (0 = current)
51 
;     +2     4   address of command line to place at PSP:0080
 51 
;     +2     4   address of command line to place at PSP:0080
52 
;     +6     4   address of an FCB to be placed at PSP:005c
 52 
;     +6     4   address of an FCB to be placed at PSP:005c
53 
;    +0Ah    4   address of an FCB to be placed at PSP:006c
 53 
;    +0Ah    4   address of an FCB to be placed at PSP:006c
54 
EXEC_PARAM_REC db 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0   ; +E0h
 54 
EXEC_PARAM_REC db 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0   ; +E0h
55 
 
 55 
 
56 
; Program to execute, preset by SvarCOM (128 bytes, ASCIIZ)
 56 
; Program to execute, preset by SvarCOM (128 bytes, ASCIIZ)
57 
EXECPROG: times 128 db 0                                     ; +EEh
 57 
EXECPROG: times 128 db 0                                     ; +EEh
58 
 
 58 
 
59 
; File where stdin and stdout should be redirected (0 = no redirection)
 59 
; File where stdin and stdout should be redirected (0 = no redirection)
60 
REDIR_INFIL:     times 128 db 0     ; +16Eh
 60 
REDIR_INFIL:     times 128 db 0     ; +16Eh
61 
REDIR_OUTFIL:    times 128 db 0     ; +1EEh
 61 
REDIR_OUTFIL:    times 128 db 0     ; +1EEh
62 
REDIR_OUTAPPEND: dw 0               ; +26Eh
 62 
REDIR_OUTAPPEND: dw 0               ; +26Eh
63 
REDIR_DEL_STDIN: db 0               ; +270h  indicates that the stdin file
 63 
REDIR_DEL_STDIN: db 0               ; +270h  indicates that the stdin file
64 
                                    ;        should be deleted (pipes). This
 64 
                                    ;        should be deleted (pipes). This
65 
                                    ;        MUST contain the 1st char of
 65 
                                    ;        MUST contain the 1st char of
66 
                                    ;        REDIR_INFIL!
 66 
                                    ;        REDIR_INFIL!
67 
 
 67 
 
68 
; CTRL+BREAK (int 23h) handler
 68 
; CTRL+BREAK (int 23h) handler
69 
; According to the TechHelp! Manual: "If you want to abort (exit to the parent
 69 
; According to the TechHelp! Manual: "If you want to abort (exit to the parent
70 
; process), then set the carry flag and return via a FAR RET. This causes DOS
 70 
; process), then set the carry flag and return via a FAR RET. This causes DOS
71 
; to perform normal cleanup and exit to the parent." (otherwise use iret)
 71 
; to perform normal cleanup and exit to the parent." (otherwise use iret)
72 
BREAK_HANDLER:            ; +271h
 72 
BREAK_HANDLER:            ; +271h
73 
stc
 73 
stc
74 
retf
 74 
retf
75 
 
 75 
 
76 
; INT 0x2E handler
 76 
; INT 0x2E handler
77 
INT2E:
 77 
INT2E:
78 
xor ax, ax
 78 
xor ax, ax
79 
iret
 79 
iret
80 
 
 80 
 
81 
skipsig:                  ; +276h
 81 
skipsig:                  ; +276h
82 
 
 82 
 
83 
; set up CS=DS=SS and point SP to my private stack buffer
 83 
; set up CS=DS=SS and point SP to my private stack buffer
84 
mov ax, cs
 84 
mov ax, cs
85 
mov ds, ax
 85 
mov ds, ax
86 
mov es, ax
 86 
mov es, ax
87 
mov ss, ax
 87 
mov ss, ax
88 
mov sp, STACKPTR
 88 
mov sp, STACKPTR
89 
 
 89 
 
90 
; set up myself as break handler (int 0x23)
 90 
; set up myself as break handler (int 0x23)
91 
mov ax, 0x2523  ; set int vector 23h
 91 
mov ax, 0x2523  ; set int vector 23h
92 
mov dx, BREAK_HANDLER
 92 
mov dx, BREAK_HANDLER
93 
int 0x21
 93 
int 0x21
94 
 
 94 
 
95 
; set up myself as int 0x2E handler ("pass command to shell")
 95 
; set up myself as int 0x2E handler ("pass command to shell")
96 
mov ax, 0x252E
 96 
mov ax, 0x252E
97 
mov dx, INT2E ; TODO do something meaningful instead of a no-op
 97 
mov dx, INT2E ; TODO do something meaningful instead of a no-op
98 
int 0x21
 98 
int 0x21
99 
 
 99 
 
100 
; revert stdin/stdout redirections (if any) to their initial state
 100 
; revert stdin/stdout redirections (if any) to their initial state
101 
call REVERT_REDIR_IF_ANY
 101 
call REVERT_REDIR_IF_ANY
102 
 
 102 
 
103 
; redirect stdin and/or stdout if required
 103 
; redirect stdin and/or stdout if required
104 
call REDIR_INOUTFILE_IF_REQUIRED
 104 
call REDIR_INOUTFILE_IF_REQUIRED
105 
 
 105 
 
106 
; should I executed command.com or a pre-set application?
 106 
; should I executed command.com or a pre-set application?
107 
or [EXECPROG], byte 0
 107 
or [EXECPROG], byte 0
108 
jz EXEC_COMMAND_COM
 108 
jz EXEC_COMMAND_COM
109 
 
 109 
 
110 
; TODO: perhaps I should call the DOS SetPSP function here? But if I do, the
 110 
; TODO: perhaps I should call the DOS SetPSP function here? But if I do, the
111 
;       int 21h, ah=50h call freezes...
 111 
;       int 21h, ah=50h call freezes...
112 
;mov ah, 0x50           ; DOS 2+ -- Set PSP
 112 
;mov ah, 0x50           ; DOS 2+ -- Set PSP
113 
;mov bx, cs
 113 
;mov bx, cs
114 
;int 0x21
 114 
;int 0x21
115 
 
 115 
 
116 
; exec an application preset (by SvarCOM) in the ExecParamRec
 116 
; exec an application preset (by SvarCOM) in the ExecParamRec
117 
mov ax, 0x4B00         ; DOS 2+ - load & execute program
 117 
mov ax, 0x4B00         ; DOS 2+ - load & execute program
118 
mov dx, EXECPROG       ; DS:DX  - ASCIZ program name (preset at PSP[already)
 118 
mov dx, EXECPROG       ; DS:DX  - ASCIZ program name (preset at PSP[already)
119 
mov bx, EXEC_PARAM_REC ; ES:BX  - parameter block pointer
 119 
mov bx, EXEC_PARAM_REC ; ES:BX  - parameter block pointer
120 
int 0x21
 120 
int 0x21
121 
mov [cs:EXECPROG], byte 0 ; do not run app again (+DS might have been changed)
 121 
mov [cs:EXECPROG], byte 0 ; do not run app again (+DS might have been changed)
122 
 
 122 
 
123 
jmp short skipsig      ; enforce valid ds/ss/etc (can be lost after int 21,4b)
 123 
jmp short skipsig      ; enforce valid ds/ss/etc (can be lost after int 21,4b)
124 
 
 124 
 
125 
EXEC_COMMAND_COM:
 125 
EXEC_COMMAND_COM:
126 
 
 126 
 
127 
; collect the exit code of previous application
 127 
; collect the exit code of previous application
128 
mov ah, 0x4D
 128 
mov ah, 0x4D
129 
int 0x21
 129 
int 0x21
130 
mov [LEXCODE], al
 130 
mov [LEXCODE], al
131 
 
 131 
 
132 
; zero out the exec param block (14 bytes)
 132 
; zero out the exec param block (14 bytes)
133 
mov al, 0              ; byte to write
 133 
mov al, 0              ; byte to write
134 
mov cx, 14             ; how many times
 134 
mov cx, 14             ; how many times
135 
mov di, EXEC_PARAM_REC ; ES:DI = destination
 135 
mov di, EXEC_PARAM_REC ; ES:DI = destination
136 
cld                    ; stosb must move forward
 136 
cld                    ; stosb must move forward
137 
rep stosb              ; repeat cx times
 137 
rep stosb              ; repeat cx times
138 
 
 138 
 
139 
; preset the default COMSPEC pointer to ES:DX (ES is already set to DS)
 139 
; preset the default COMSPEC pointer to ES:DX (ES is already set to DS)
140 
mov dx, COMSPECBOOT
 140 
mov dx, COMSPECBOOT
141 
 
 141 
 
142 
; do I have a valid COMSPEC?
 142 
; do I have a valid COMSPEC?
143 
or [COMSPECPTR], word 0
 143 
or [COMSPECPTR], word 0
144 
jz USEDEFAULTCOMSPEC
 144 
jz USEDEFAULTCOMSPEC
145 
; set ES:DX to actual COMSPEC (in env segment)
 145 
; set ES:DX to actual COMSPEC (in env segment)
146 
mov es, [PSP_ENVSEG]
 146 
mov es, [PSP_ENVSEG]
147 
mov dx, [COMSPECPTR]
 147 
mov dx, [COMSPECPTR]
148 
USEDEFAULTCOMSPEC:
 148 
USEDEFAULTCOMSPEC:
149 
 
 149 
 
150 
; prepare the exec param block
 150 
; prepare the exec param block
151 
mov ax, [PSP_ENVSEG]
 151 
mov ax, [PSP_ENVSEG]
152 
mov [EXEC_PARAM_REC], ax
 152 
mov [EXEC_PARAM_REC], ax
153 
mov [EXEC_PARAM_REC+2], word CMDTAIL
 153 
mov [EXEC_PARAM_REC+2], word CMDTAIL
154 
mov [EXEC_PARAM_REC+4], cs
 154 
mov [EXEC_PARAM_REC+4], cs
155 
 
 155 
 
156 
; execute command.com
 156 
; execute command.com
157 
mov ax, 0x4B00         ; DOS 2+ - load & execute program
 157 
mov ax, 0x4B00         ; DOS 2+ - load & execute program
158 
push es                ;
 158 
push es                ;
159 
pop ds                 ;
 159 
pop ds                 ;
160 
;mov dx, COMSPEC       ; DS:DX  - ASCIZ program name (preset already)
 160 
;mov dx, COMSPEC       ; DS:DX  - ASCIZ program name (preset already)
161 
push cs
 161 
push cs
162 
pop es
 162 
pop es
163 
mov bx, EXEC_PARAM_REC ; ES:BX  - parameter block pointer
 163 
mov bx, EXEC_PARAM_REC ; ES:BX  - parameter block pointer
164 
int 0x21
 164 
int 0x21
165 
 
 165 
 
166 
; if all went well, jump back to start
 166 
; if all went well, jump back to start
167 
jnc skipsig
 167 
jnc skipsig
168 
 
 168 
 
169 
; restore DS=CS
 169 
; restore DS=CS
170 
mov bx, cs
 170 
mov bx, cs
171 
mov ds, bx
 171 
mov ds, bx
172 
 
 172 
 
173 
; update error string so it contains the error number
 173 
; update error string so it contains the error number
174 
add al, '0'
 174 
add al, '0'
175 
mov [ERRLOAD + 4], al
 175 
mov [ERRLOAD + 4], al
176 
 
 176 
 
177 
; display error message
 177 
; display error message
178 
mov ah, 0x09
 178 
mov ah, 0x09
179 
mov dx, ERRLOAD
 179 
mov dx, ERRLOAD
180 
int 0x21
 180 
int 0x21
181 
 
 181 
 
182 
; wait for keypress
 182 
; wait for keypress
183 
mov ah, 0x08
 183 
mov ah, 0x08
184 
int 0x21
 184 
int 0x21
185 
 
 185 
 
186 
; back to program start
 186 
; back to program start
187 
jmp skipsig
 187 
jmp skipsig
188 
 
 188 
 
189 
; command.com tail arguments, in PSP format: length byte followed by args and
 189 
; command.com tail arguments, in PSP format: length byte followed by args and
190 
; terminated with \r) - a single 0x0A byte is passed so SvarCOM knows it is
 190 
; terminated with \r) - a single 0x0A byte is passed so SvarCOM knows it is
191 
; called as respawn (as opposed to being invoked as a normal application)
 191 
; called as respawn (as opposed to being invoked as a normal application)
192 
; this allows multiple copies of SvarCOM to stack upon each other.
 192 
; this allows multiple copies of SvarCOM to stack upon each other.
193 
CMDTAIL db 0x01, 0x0A, 0x0D
 193 
CMDTAIL db 0x01, 0x0A, 0x0D
194 
 
 194 
 
195 
ERRLOAD db "ERR x, FAILED TO LOAD COMMAND.COM", 13, 10, '$'
 195 
ERRLOAD db "ERR x, FAILED TO LOAD COMMAND.COM", 13, 10, '$'
196 
 
 196 
 
197 
; variables used to revert stdin/stdout to their initial state
 197 
; variables used to revert stdin/stdout to their initial state
198 
OLD_STDOUT dw 0xffff
 198 
OLD_STDOUT dw 0xffff
199 
OLD_STDIN  dw 0xffff
 199 
OLD_STDIN  dw 0xffff
200 
 
 200 
 
201 
 
 201 
 
202 
; ****************************************************************************
 202 
; ****************************************************************************
203 
; *** ROUTINES ***************************************************************
 203 
; *** ROUTINES ***************************************************************
204 
; ****************************************************************************
 204 
; ****************************************************************************
205 
 
 205 
 
206 
; ----------------------------------------------------------------------------
 206 
; ----------------------------------------------------------------------------
207 
; revert stdin/stdout redirections (if any) to their initial state
 207 
; revert stdin/stdout redirections (if any) to their initial state
208 
REVERT_REDIR_IF_ANY:
 208 
REVERT_REDIR_IF_ANY:
209 
; is stdout redirected?
 209 
; is stdout redirected?
210 
mov bx, [OLD_STDOUT]
 210 
mov bx, [OLD_STDOUT]
211 
cmp bx, 0xffff
 211 
cmp bx, 0xffff
212 
je STDOUT_DONE
 212 
je STDOUT_DONE
213 
; revert the stdout handle (dst in BX already)
 213 
; revert the stdout handle (dst in BX already)
214 
mov cx, 1        ; src handle (1=stdout)
 214 
mov cx, 1        ; src handle (1=stdout)
215 
mov ah, 0x46     ; redirect a handle
 215 
mov ah, 0x46     ; redirect a handle
216 
int 0x21
 216 
int 0x21
217 
; close the old handle (still in bx)
 217 
; close the old handle (still in bx)
218 
mov ah, 0x3e
 218 
mov ah, 0x3e
219 
int 0x21
 219 
int 0x21
220 
mov [OLD_STDOUT], word 0xffff ; mark stdout as "not redirected"
 220 
mov [OLD_STDOUT], word 0xffff ; mark stdout as "not redirected"
221 
STDOUT_DONE:
 221 
STDOUT_DONE:
222 
 
 222 
 
223 
; is stdin redirected?
 223 
; is stdin redirected?
224 
mov bx, [OLD_STDIN]
 224 
mov bx, [OLD_STDIN]
225 
cmp bx, 0xffff
 225 
cmp bx, 0xffff
226 
je STDIN_DONE
 226 
je STDIN_DONE
227 
; revert the stdin handle (dst in BX already)
 227 
; revert the stdin handle (dst in BX already)
228 
xor cx, cx       ; src handle (0=stdin)
 228 
xor cx, cx       ; src handle (0=stdin)
229 
mov ah, 0x46     ; redirect a handle
 229 
mov ah, 0x46     ; redirect a handle
230 
int 0x21
 230 
int 0x21
231 
; close the old handle (still in bx)
 231 
; close the old handle (still in bx)
232 
mov ah, 0x3e
 232 
mov ah, 0x3e
233 
int 0x21
 233 
int 0x21
234 
mov [OLD_STDIN], word 0xffff ; mark stdin as "not redirected"
 234 
mov [OLD_STDIN], word 0xffff ; mark stdin as "not redirected"
235 
 
 235 
 
236 
; delete stdin file if required
 236 
; delete stdin file if required
237 
cmp [REDIR_DEL_STDIN], byte 0
 237 
cmp [REDIR_DEL_STDIN], byte 0
238 
je STDIN_DONE
 238 
je STDIN_DONE
239 
; revert the original file and delete it
 239 
; revert the original file and delete it
240 
mov ah, [REDIR_DEL_STDIN]
 240 
mov ah, [REDIR_DEL_STDIN]
241 
mov [REDIR_INFIL], ah
 241 
mov [REDIR_INFIL], ah
242 
mov ah, 0x41     ; DOS 2+ - delete file pointed at by DS:DX
 242 
mov ah, 0x41     ; DOS 2+ - delete file pointed at by DS:DX
243 
mov dx, REDIR_INFIL
 243 
mov dx, REDIR_INFIL
244 
int 0x21
 244 
int 0x21
245 
mov [REDIR_INFIL], byte 0
 245 
mov [REDIR_INFIL], byte 0
246 
mov [REDIR_DEL_STDIN], byte 0
 246 
mov [REDIR_DEL_STDIN], byte 0
247 
 
 247 
 
248 
STDIN_DONE:
 248 
STDIN_DONE:
249 
 
 249 
 
250 
ret
 250 
ret
251 
; ----------------------------------------------------------------------------
 251 
; ----------------------------------------------------------------------------
252 
 
 252 
 
253 
 
 253 
 
254 
; ----------------------------------------------------------------------------
 254 
; ----------------------------------------------------------------------------
255 
; redirect stdout if REDIR_OUTFIL points to something
 255 
; redirect stdout if REDIR_OUTFIL points to something
256 
REDIR_INOUTFILE_IF_REQUIRED:
 256 
REDIR_INOUTFILE_IF_REQUIRED:
257 
cmp [REDIR_OUTFIL], byte 0
 257 
cmp [REDIR_OUTFIL], byte 0
258 
je NO_STDOUT_REDIR
 258 
je NO_STDOUT_REDIR
259 
mov si, REDIR_OUTFIL   ; si = output file
 259 
mov si, REDIR_OUTFIL   ; si = output file
260 
mov ax, 0x6c00         ; Extended Open/Create
 260 
mov ax, 0x6c00         ; Extended Open/Create
261 
mov bx, 1              ; access mode (0=read, 1=write, 2=r+w)
 261 
mov bx, 1              ; access mode (0=read, 1=write, 2=r+w)
262 
xor cx, cx             ; file attribs when(if) file is created (0=normal)
 262 
xor cx, cx             ; file attribs when(if) file is created (0=normal)
263 
mov dx, [REDIR_OUTAPPEND] ; action if file exist (0x11=open, 0x12=truncate)
 263 
mov dx, [REDIR_OUTAPPEND] ; action if file exist (0x11=open, 0x12=truncate)
264 
int 0x21               ; ax=handle on success (CF clear)
 264 
int 0x21               ; ax=handle on success (CF clear)
265 
mov [REDIR_OUTFIL], byte 0
 265 
mov [REDIR_OUTFIL], byte 0
266 
jc NO_STDOUT_REDIR     ; TODO: abort with an error message instead
 266 
jc NO_STDOUT_REDIR     ; TODO: abort with an error message instead
267 
 
 267 
 
268 
; jump to end of file if flag was 0x11 (required for >> redirections)
 268 
; jump to end of file if flag was 0x11 (required for >> redirections)
269 
cmp [REDIR_OUTAPPEND], word 0x11
 269 
cmp [REDIR_OUTAPPEND], word 0x11
270 
jne SKIP_JMPEOF
 270 
jne SKIP_JMPEOF
271 
mov bx, ax
 271 
mov bx, ax
272 
mov ax, 0x4202         ; jump to position EOF - CX:DX in handle BX
 272 
mov ax, 0x4202         ; jump to position EOF - CX:DX in handle BX
273 
xor cx, cx
 273 
xor cx, cx
274 
xor dx, dx
 274 
xor dx, dx
275 
int 0x21
 275 
int 0x21
276 
mov ax, bx             ; put my handle back in ax, as expected by later code
 276 
mov ax, bx             ; put my handle back in ax, as expected by later code
277 
SKIP_JMPEOF:
 277 
SKIP_JMPEOF:
278 
 
 278 
 
279 
; duplicate current stdout so I can revert it later
 279 
; duplicate current stdout so I can revert it later
280 
push ax                ; save my file handle in stack
 280 
push ax                ; save my file handle in stack
281 
mov ah, 0x45           ; duplicate file handle BX
 281 
mov ah, 0x45           ; duplicate file handle BX
282 
mov bx, 1              ; 1 = stdout
 282 
mov bx, 1              ; 1 = stdout
283 
int 0x21               ; ax=new (duplicated) file handle
 283 
int 0x21               ; ax=new (duplicated) file handle
284 
mov [OLD_STDOUT], ax   ; save the old handle in memory
 284 
mov [OLD_STDOUT], ax   ; save the old handle in memory
285 
 
 285 
 
286 
; redirect stdout to my file
 286 
; redirect stdout to my file
287 
pop bx                 ; dst handle
 287 
pop bx                 ; dst handle
288 
mov cx, 1              ; src handle (1=stdout)
 288 
mov cx, 1              ; src handle (1=stdout)
289 
mov ah, 0x46           ; "redirect a handle"
 289 
mov ah, 0x46           ; "redirect a handle"
290 
int 0x21
 290 
int 0x21
291 
 
 291 
 
292 
; close the original file handle, I no longer need it
 292 
; close the original file handle, I no longer need it
293 
mov ah, 0x3e           ; close a file handle (handle in BX)
 293 
mov ah, 0x3e           ; close a file handle (handle in BX)
294 
int 0x21
 294 
int 0x21
295 
NO_STDOUT_REDIR:
 295 
NO_STDOUT_REDIR:
296 
 
 296 
 
297 
; *** redirect stdin if REDIR_INFIL points to something ***
 297 
; *** redirect stdin if REDIR_INFIL points to something ***
298 
cmp [REDIR_INFIL], byte 0
 298 
cmp [REDIR_INFIL], byte 0
299 
je NO_STDIN_REDIR
 299 
je NO_STDIN_REDIR
300 
mov dx, REDIR_INFIL    ; dx:dx = file
 300 
mov dx, REDIR_INFIL    ; dx:dx = file
301 
mov ax, 0x3d00         ; open file for read
 301 
mov ax, 0x3d00         ; open file for read
302 
int 0x21               ; ax=handle on success (CF clear)
 302 
int 0x21               ; ax=handle on success (CF clear)
303 
mov [REDIR_INFIL], byte 0
 303 
mov [REDIR_INFIL], byte 0
304 
jc NO_STDIN_REDIR      ; TODO: abort with an error message instead
 304 
jc NO_STDIN_REDIR      ; TODO: abort with an error message instead
305 
 
 305 
 
306 
; duplicate current stdin so I can revert it later
 306 
; duplicate current stdin so I can revert it later
307 
push ax                ; save my file handle in stack
 307 
push ax                ; save my file handle in stack
308 
mov ah, 0x45           ; duplicate file handle BX
 308 
mov ah, 0x45           ; duplicate file handle BX
309 
xor bx, bx             ; 0=stdin
 309 
xor bx, bx             ; 0=stdin
310 
int 0x21               ; ax=new (duplicated) file handle
 310 
int 0x21               ; ax=new (duplicated) file handle
311 
mov [OLD_STDIN], ax    ; save the old handle in memory
 311 
mov [OLD_STDIN], ax    ; save the old handle in memory
312 
 
 312 
 
313 
; redirect stdout to my file
 313 
; redirect stdout to my file
314 
pop bx                 ; dst handle
 314 
pop bx                 ; dst handle
315 
xor cx, cx             ; src handle (0=stdin)
 315 
xor cx, cx             ; src handle (0=stdin)
316 
mov ah, 0x46           ; "redirect a handle"
 316 
mov ah, 0x46           ; "redirect a handle"
317 
int 0x21
 317 
int 0x21
318 
 
 318 
 
319 
; close the original file handle, I no longer need it
 319 
; close the original file handle, I no longer need it
320 
mov ah, 0x3e           ; close a file handle (handle in BX)
 320 
mov ah, 0x3e           ; close a file handle (handle in BX)
321 
int 0x21
 321 
int 0x21
322 
NO_STDIN_REDIR:
 322 
NO_STDIN_REDIR:
323 
ret
 323 
ret
324 
; ----------------------------------------------------------------------------
 324 
; ----------------------------------------------------------------------------
325 
 
 325