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Rev 1881 Rev 1986
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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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test [COMSPECPTR], word 0xffff
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jz USEDEFAULTCOMSPEC
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jz USEDEFAULTCOMSPEC
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; set ES:DX to actual COMSPEC (in env segment)
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; set ES:DX to actual COMSPEC (in env segment)
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mov es, [PSP_ENVSEG]
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mov es, [PSP_ENVSEG]
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mov dx, [COMSPECPTR]
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mov dx, [COMSPECPTR]
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USEDEFAULTCOMSPEC:
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USEDEFAULTCOMSPEC:
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mov ax, [PSP_ENVSEG]
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mov ax, [PSP_ENVSEG]
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mov [EXEC_PARAM_REC], ax
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mov [EXEC_PARAM_REC], ax
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mov [EXEC_PARAM_REC+2], word CMDTAIL
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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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mov [EXEC_PARAM_REC+4], cs
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- 
 
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mov si, dx             ; preset si to command in case I will display it in the
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                       ; err message (DX might be destroyed by int 21h,AH=4Bh)
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; execute command.com
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; execute command.com
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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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push es                ;
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push es                ;
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pop ds                 ;
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pop ds                 ;
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;mov dx, COMSPEC       ; DS:DX  - ASCIZ program name (preset already)
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;mov dx, COMSPEC       ; DS:DX  - ASCIZ program name (preset already)
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; save error code into cl
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; save error code into cl
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mov cl, al
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mov cl, al
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; display program name (in DS:DX), but first replace its nul terminator by a $
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; display program name (in DS:DX), but first replace its nul terminator by a $
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mov si, dx
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;mov si, dx           ; done already before the exec attempt
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PARSENEXTBYTE:
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PARSENEXTBYTE:
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lodsb ; load byte at DS:SI into AL and inc SI (direction flag is clear already)
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lodsb ; load byte at DS:SI into AL and inc SI (direction flag is clear already)
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test al, al  ; is zero yet?
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test al, al  ; is zero yet?
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jnz PARSENEXTBYTE
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jnz PARSENEXTBYTE
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mov [si], byte '$'   ; replace the nul terminator by a $ and display it
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mov [si], byte '$'   ; replace the nul terminator by a $ and display it
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;          50H (set PSP address)
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;          50H (set PSP address)
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;          51H and 62H (query PSP address)
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;          51H and 62H (query PSP address)
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;          59H (get extended error information)
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;          59H (get extended error information)
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;
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;
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; =============================================================================
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; =============================================================================
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HANDLER_24H:    ; +46Ch
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HANDLER_24H:    ; +46Dh
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; save registers so I can restore them later. AX does not require saving.
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; save registers so I can restore them later. AX does not require saving.
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; Microsoft documentation says:
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; Microsoft documentation says:
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;  "When it is desired to ignore an error, the same registers must be preserved
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;  "When it is desired to ignore an error, the same registers must be preserved
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;   as when it is desired to retry the operation (SS,SP,DS,BX,CX,DX)."
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;   as when it is desired to retry the operation (SS,SP,DS,BX,CX,DX)."