Subversion Repositories SvarDOS

;

; rmod - resident module of the SvarCOM command interpreter

;

; Copyright (C) 2021 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  this acts also as a guardval to detect stack overflows

; 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

; exit code of last application

LEXCODE  dw 0    ; +4Ah

; 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  ; +4Ch

; 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 ; +4Eh

; 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   ; +5Dh

; Program to execute, preset by SvarCOM (128 bytes, ASCIIZ)  ; +6Bh

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

skipsig:         ; +EBh

; 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

; 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

xor ah, ah          ; clear out termination status, I only want the exit code

mov [LEXCODE], ax

; 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, '$'