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;
; rmod - resident module of the SvarCOM command interpreter (NASM code)
;
; Copyright (C) 2021-2022 Mateusz Viste
; MIT license
;
; this is installed in memory by the transient part of SvarCOM. it has only
; two jobs: providing a resident buffer for command history, environment, etc
; and respawning COMMAND.COM whenever necessary.
CPU 8086
org 0x100
PSP_ENVSEG equ 0x2C
section .text ; all goes into code segment
; offset
SIG1 dw 0x1983 ; +0
SIG2 dw 0x1985 ; +2
SIG3 dw 0x2017 ; +4
SIG4 dw 0x2019 ; +6 acts also as a guardval to detect severe stack overflows
; Buffer used to remember previous command, when SvarCOM calls the buffered
; input service at INT 21h,AH=0x0A.
; This buffer is right before the stack, so in case of a stack overflow event
; (for example because of a "too ambitious" TSR) only this buffer is damaged,
; and can be invalidated without much harm. To detect such damage, SvarCOM's
; transient part is appending a signature at the end of the buffer.
INPUTBUF: times 132 db 0 ; 130 bytes for the input buffer + 2 for signature
; DOS int 21h functions that I use require at least 40 bytes of stack under
; DOS-C (FreeDOS) kernel, so here I reserve 64 bytes juste to be sure
STACKBUF db "XXX SVARCOM RMOD BY MATEUSZ VISTE XXXXXXXXXXXXXXXXXXXXXXXXXXXX"
STACKPTR dw 0
; offset of the COMSPEC variable in the environment block, 0 means "use
; boot drive". this value is patched by the transient part of COMMAND.COM
COMSPECPTR dw 0 ; +CEh
; fallback COMSPEC string used if no COMPSEC is present in the environment
; drive. drive is patched by the transient part of COMMAND.COM
COMSPECBOOT db "@:\COMMAND.COM", 0 ; +D0h
; exit code of last application
LEXCODE db 0 ; +DFh
; ExecParamRec used by INT 21h, AX=4b00 (load and execute program), 14 bytes:
; offset size content
; +0 2 segment of environment for child (0 = current)
; +2 4 address of command line to place at PSP:0080
; +6 4 address of an FCB to be placed at PSP:005c
; +0Ah 4 address of an FCB to be placed at PSP:006c
EXEC_PARAM_REC db 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 ; +E0h
; Program to execute, preset by SvarCOM (128 bytes, ASCIIZ)
EXECPROG: times 128 db 0 ; +EEh
; File where stdin and stdout should be redirected (0 = no redirection)
REDIR_INFIL: times 128 db 0 ; +16Eh
REDIR_OUTFIL: times 128 db 0 ; +1EEh
REDIR_OUTAPPEND: dw 0 ; +26Eh
REDIR_DEL_STDIN: db 0 ; +270h indicates that the stdin file
; should be deleted (pipes). This
; MUST contain the 1st char of
; REDIR_INFIL!
; CTRL+BREAK (int 23h) handler
; According to the TechHelp! Manual: "If you want to abort (exit to the parent
; process), then set the carry flag and return via a FAR RET. This causes DOS
; to perform normal cleanup and exit to the parent." (otherwise use iret)
BREAK_HANDLER: ; +271h
stc
retf
; INT 0x2E handler
INT2E:
xor ax, ax
iret
skipsig: ; +276h
; set up CS=DS=SS and point SP to my private stack buffer
mov ax, cs
mov ds, ax
mov es, ax
mov ss, ax
mov sp, STACKPTR
; set up myself as break handler (int 0x23)
mov ax, 0x2523 ; set int vector 23h
mov dx, BREAK_HANDLER
int 0x21
; set up myself as int 0x2E handler ("pass command to shell")
mov ax, 0x252E
mov dx, INT2E ; TODO do something meaningful instead of a no-op
int 0x21
; revert stdin/stdout redirections (if any) to their initial state
call REVERT_REDIR_IF_ANY
; redirect stdin and/or stdout if required
call REDIR_INOUTFILE_IF_REQUIRED
; should I executed command.com or a pre-set application?
or [EXECPROG], byte 0
jz EXEC_COMMAND_COM
; TODO: perhaps I should call the DOS SetPSP function here? But if I do, the
; int 21h, ah=50h call freezes...
;mov ah, 0x50 ; DOS 2+ -- Set PSP
;mov bx, cs
;int 0x21
; exec an application preset (by SvarCOM) in the ExecParamRec
mov ax, 0x4B00 ; DOS 2+ - load & execute program
mov dx, EXECPROG ; DS:DX - ASCIZ program name (preset at PSP[already)
mov bx, EXEC_PARAM_REC ; ES:BX - parameter block pointer
int 0x21
mov [cs:EXECPROG], byte 0 ; do not run app again (+DS might have been changed)
jmp short skipsig ; enforce valid ds/ss/etc (can be lost after int 21,4b)
EXEC_COMMAND_COM:
; collect the exit code of previous application
mov ah, 0x4D
int 0x21
mov [LEXCODE], al
; zero out the exec param block (14 bytes)
mov al, 0 ; byte to write
mov cx, 14 ; how many times
mov di, EXEC_PARAM_REC ; ES:DI = destination
cld ; stosb must move forward
rep stosb ; repeat cx times
; preset the default COMSPEC pointer to ES:DX (ES is already set to DS)
mov dx, COMSPECBOOT
; do I have a valid COMSPEC?
or [COMSPECPTR], word 0
jz USEDEFAULTCOMSPEC
; set ES:DX to actual COMSPEC (in env segment)
mov es, [PSP_ENVSEG]
mov dx, [COMSPECPTR]
USEDEFAULTCOMSPEC:
; prepare the exec param block
mov ax, [PSP_ENVSEG]
mov [EXEC_PARAM_REC], ax
mov [EXEC_PARAM_REC+2], word CMDTAIL
mov [EXEC_PARAM_REC+4], cs
; execute command.com
mov ax, 0x4B00 ; DOS 2+ - load & execute program
push es ;
pop ds ;
;mov dx, COMSPEC ; DS:DX - ASCIZ program name (preset already)
push cs
pop es
mov bx, EXEC_PARAM_REC ; ES:BX - parameter block pointer
int 0x21
; if all went well, jump back to start
jnc skipsig
; restore DS=CS
mov bx, cs
mov ds, bx
; update error string so it contains the error number
add al, '0'
mov [ERRLOAD + 4], al
; display error message
mov ah, 0x09
mov dx, ERRLOAD
int 0x21
; wait for keypress
mov ah, 0x08
int 0x21
; back to program start
jmp skipsig
; command.com tail arguments, in PSP format: length byte followed by args and
; terminated with \r) - a single 0x0A byte is passed so SvarCOM knows it is
; called as respawn (as opposed to being invoked as a normal application)
; this allows multiple copies of SvarCOM to stack upon each other.
CMDTAIL db 0x01, 0x0A, 0x0D
ERRLOAD db "ERR x, FAILED TO LOAD COMMAND.COM", 13, 10, '$'
; variables used to revert stdin/stdout to their initial state
OLD_STDOUT dw 0xffff
OLD_STDIN dw 0xffff
; ****************************************************************************
; *** ROUTINES ***************************************************************
; ****************************************************************************
; ----------------------------------------------------------------------------
; revert stdin/stdout redirections (if any) to their initial state
REVERT_REDIR_IF_ANY:
; is stdout redirected?
mov bx, [OLD_STDOUT]
cmp bx, 0xffff
je STDOUT_DONE
; revert the stdout handle (dst in BX already)
mov cx, 1 ; src handle (1=stdout)
mov ah, 0x46 ; redirect a handle
int 0x21
; close the old handle (still in bx)
mov ah, 0x3e
int 0x21
mov [OLD_STDOUT], word 0xffff ; mark stdout as "not redirected"
STDOUT_DONE:
; is stdin redirected?
mov bx, [OLD_STDIN]
cmp bx, 0xffff
je STDIN_DONE
; revert the stdin handle (dst in BX already)
xor cx, cx ; src handle (0=stdin)
mov ah, 0x46 ; redirect a handle
int 0x21
; close the old handle (still in bx)
mov ah, 0x3e
int 0x21
mov [OLD_STDIN], word 0xffff ; mark stdin as "not redirected"
; delete stdin file if required
cmp [REDIR_DEL_STDIN], byte 0
je STDIN_DONE
; revert the original file and delete it
mov ah, [REDIR_DEL_STDIN]
mov [REDIR_INFIL], ah
mov ah, 0x41 ; DOS 2+ - delete file pointed at by DS:DX
mov dx, REDIR_INFIL
int 0x21
mov [REDIR_INFIL], byte 0
mov [REDIR_DEL_STDIN], byte 0
STDIN_DONE:
ret
; ----------------------------------------------------------------------------
; ----------------------------------------------------------------------------
; redirect stdout if REDIR_OUTFIL points to something
REDIR_INOUTFILE_IF_REQUIRED:
cmp [REDIR_OUTFIL], byte 0
je NO_STDOUT_REDIR
mov si, REDIR_OUTFIL ; si = output file
mov ax, 0x6c00 ; Extended Open/Create
mov bx, 1 ; access mode (0=read, 1=write, 2=r+w)
xor cx, cx ; file attribs when(if) file is created (0=normal)
mov dx, [REDIR_OUTAPPEND] ; action if file exist (0x11=open, 0x12=truncate)
int 0x21 ; ax=handle on success (CF clear)
mov [REDIR_OUTFIL], byte 0
jc NO_STDOUT_REDIR ; TODO: abort with an error message instead
; jump to end of file if flag was 0x11 (required for >> redirections)
cmp [REDIR_OUTAPPEND], word 0x11
jne SKIP_JMPEOF
mov bx, ax
mov ax, 0x4202 ; jump to position EOF - CX:DX in handle BX
xor cx, cx
xor dx, dx
int 0x21
mov ax, bx ; put my handle back in ax, as expected by later code
SKIP_JMPEOF:
; duplicate current stdout so I can revert it later
push ax ; save my file handle in stack
mov ah, 0x45 ; duplicate file handle BX
mov bx, 1 ; 1 = stdout
int 0x21 ; ax=new (duplicated) file handle
mov [OLD_STDOUT], ax ; save the old handle in memory
; redirect stdout to my file
pop bx ; dst handle
mov cx, 1 ; src handle (1=stdout)
mov ah, 0x46 ; "redirect a handle"
int 0x21
; close the original file handle, I no longer need it
mov ah, 0x3e ; close a file handle (handle in BX)
int 0x21
NO_STDOUT_REDIR:
; *** redirect stdin if REDIR_INFIL points to something ***
cmp [REDIR_INFIL], byte 0
je NO_STDIN_REDIR
mov dx, REDIR_INFIL ; dx:dx = file
mov ax, 0x3d00 ; open file for read
int 0x21 ; ax=handle on success (CF clear)
mov [REDIR_INFIL], byte 0
jc NO_STDIN_REDIR ; TODO: abort with an error message instead
; duplicate current stdin so I can revert it later
push ax ; save my file handle in stack
mov ah, 0x45 ; duplicate file handle BX
xor bx, bx ; 0=stdin
int 0x21 ; ax=new (duplicated) file handle
mov [OLD_STDIN], ax ; save the old handle in memory
; redirect stdout to my file
pop bx ; dst handle
xor cx, cx ; src handle (0=stdin)
mov ah, 0x46 ; "redirect a handle"
int 0x21
; close the original file handle, I no longer need it
mov ah, 0x3e ; close a file handle (handle in BX)
int 0x21
NO_STDIN_REDIR:
ret
; ----------------------------------------------------------------------------