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Rev 1713 Rev 1730
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;
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;
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; rmod - resident module of the SvarCOM command interpreter (NASM code)
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; rmod - resident module of the SvarCOM command interpreter (NASM code)
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;
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;
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; Copyright (C) 2021-2022 Mateusz Viste
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; Copyright (C) 2021-2024 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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REDIR_DEL_STDIN: db 0               ; +270h  indicates that the stdin file
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REDIR_DEL_STDIN: db 0               ; +270h  indicates that the stdin file
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                                    ;        should be deleted (pipes). This
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                                    ;        should be deleted (pipes). This
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                                    ;        MUST contain the 1st char of
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                                    ;        MUST contain the 1st char of
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                                    ;        REDIR_INFIL!
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                                    ;        REDIR_INFIL!
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- 
 
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EXEC_LH: db 0                       ; +271h  EXECPROG to be loaded high?
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ORIG_UMBLINKSTATE: db 0             ; +272h
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ORIG_ALLOCSTRAT: db 0               ; +273h
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; CTRL+BREAK (int 23h) handler
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; 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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; 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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; 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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; to perform normal cleanup and exit to the parent." (otherwise use iret)
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BREAK_HANDLER:            ; +271h
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BREAK_HANDLER:            ; +274h
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stc
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stc
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retf
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retf
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; INT 0x2E handler
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; INT 0x2E handler
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INT2E:
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INT2E:
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xor ax, ax
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xor ax, ax
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iret
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iret
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skipsig:                  ; +276h
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skipsig:                  ; +279h
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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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;       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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- 
 
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; LOADHIGH?
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cmp [EXEC_LH], byte 0
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je NO_LOADHIGH
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; SAVE CURRENT UMB LINK STATE
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mov ax, 0x5802  ; GET UMB LINK STATE
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int 0x21
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mov [ORIG_UMBLINKSTATE], al
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; SAVE CURRENT ALLOCATION STRATEGY
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mov ax, 0x5800
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int 0x21
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mov [ORIG_ALLOCSTRAT], al
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; LOADHIGH: link in the UMB memory chain for enabling high-memory allocation
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;           (and save initial status on stack)
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mov ax, 0x5803  ; SET UMB LINK STATE */
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mov bx, 1
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int 0x21
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; set strategy to 'last fit, try high then low memory'
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mov ax, 0x5801
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mov bx, 0x0082
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int 0x21
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NO_LOADHIGH:
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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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- 
 
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; go to start if nothing else to do (this will enforce valid ds/ss/etc)
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cmp [cs:EXEC_LH], byte 0
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je skipsig
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; restore UMB link state and alloc strategy to original values (but make sure
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; to base it on CS since DS might have been trashed by the program)
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mov ax, 0x5803
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xor bx, bx
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mov bl, [cs:ORIG_UMBLINKSTATE]
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int 0x21
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; restore original memory allocation strategy
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mov ax, 0x5801
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mov bl, [cs:ORIG_ALLOCSTRAT]
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int 0x21
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; turn off the LH flag
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mov [cs:EXEC_LH], byte 0
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- 
 
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jmp short skipsig      ; enforce valid ds/ss/etc (can be lost after int 21,4b)
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jmp 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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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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- 
 
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; zero out the LH flag
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mov [EXEC_LH], byte 0
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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