Assembly language programming problem in dos system initialization












1















I have a little problem in understanding that why we are storing ES and AX values in STACK. This is mentioned in lines "line1" and "line2", respectively.



EXTERN RE_INIT:FAR

ASSUME ES:SYSINITSEG

PUSH ES
MOV AX, OFFSET SYSIN
PUSH AX

RE_INIT PROC FAR
RET
RE_INIT ENDP

;
;MOV THE DOS TO IT'S PROPER LOCATION
;

SYSIN:
.........code continues.........









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  • 1





    Because the value of ES is not known until runtime. One way around this is to push the segment you want to jump to, followed by the offset and then issuing a FAR RETURN. The effect is doing a runtime FAR JMP to SYSIN. It sets CS to what ES was and the IP (offset) of SYSIN. It is jumping to a piece of relocated code in a different segment but the same offset (SYSIN).

    – Michael Petch
    Dec 30 '18 at 5:31











  • Thanks Michael, now I have understood the complete logic. Thank

    – Vstbutwhy
    Dec 30 '18 at 6:48
















1















I have a little problem in understanding that why we are storing ES and AX values in STACK. This is mentioned in lines "line1" and "line2", respectively.



EXTERN RE_INIT:FAR

ASSUME ES:SYSINITSEG

PUSH ES
MOV AX, OFFSET SYSIN
PUSH AX

RE_INIT PROC FAR
RET
RE_INIT ENDP

;
;MOV THE DOS TO IT'S PROPER LOCATION
;

SYSIN:
.........code continues.........









share|improve this question




















  • 1





    Because the value of ES is not known until runtime. One way around this is to push the segment you want to jump to, followed by the offset and then issuing a FAR RETURN. The effect is doing a runtime FAR JMP to SYSIN. It sets CS to what ES was and the IP (offset) of SYSIN. It is jumping to a piece of relocated code in a different segment but the same offset (SYSIN).

    – Michael Petch
    Dec 30 '18 at 5:31











  • Thanks Michael, now I have understood the complete logic. Thank

    – Vstbutwhy
    Dec 30 '18 at 6:48














1












1








1








I have a little problem in understanding that why we are storing ES and AX values in STACK. This is mentioned in lines "line1" and "line2", respectively.



EXTERN RE_INIT:FAR

ASSUME ES:SYSINITSEG

PUSH ES
MOV AX, OFFSET SYSIN
PUSH AX

RE_INIT PROC FAR
RET
RE_INIT ENDP

;
;MOV THE DOS TO IT'S PROPER LOCATION
;

SYSIN:
.........code continues.........









share|improve this question
















I have a little problem in understanding that why we are storing ES and AX values in STACK. This is mentioned in lines "line1" and "line2", respectively.



EXTERN RE_INIT:FAR

ASSUME ES:SYSINITSEG

PUSH ES
MOV AX, OFFSET SYSIN
PUSH AX

RE_INIT PROC FAR
RET
RE_INIT ENDP

;
;MOV THE DOS TO IT'S PROPER LOCATION
;

SYSIN:
.........code continues.........






assembly dos masm x86-16






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share|improve this question













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edited Dec 30 '18 at 5:33









Michael Petch

25.3k556101




25.3k556101










asked Dec 30 '18 at 5:17









VstbutwhyVstbutwhy

173




173








  • 1





    Because the value of ES is not known until runtime. One way around this is to push the segment you want to jump to, followed by the offset and then issuing a FAR RETURN. The effect is doing a runtime FAR JMP to SYSIN. It sets CS to what ES was and the IP (offset) of SYSIN. It is jumping to a piece of relocated code in a different segment but the same offset (SYSIN).

    – Michael Petch
    Dec 30 '18 at 5:31











  • Thanks Michael, now I have understood the complete logic. Thank

    – Vstbutwhy
    Dec 30 '18 at 6:48














  • 1





    Because the value of ES is not known until runtime. One way around this is to push the segment you want to jump to, followed by the offset and then issuing a FAR RETURN. The effect is doing a runtime FAR JMP to SYSIN. It sets CS to what ES was and the IP (offset) of SYSIN. It is jumping to a piece of relocated code in a different segment but the same offset (SYSIN).

    – Michael Petch
    Dec 30 '18 at 5:31











  • Thanks Michael, now I have understood the complete logic. Thank

    – Vstbutwhy
    Dec 30 '18 at 6:48








1




1





Because the value of ES is not known until runtime. One way around this is to push the segment you want to jump to, followed by the offset and then issuing a FAR RETURN. The effect is doing a runtime FAR JMP to SYSIN. It sets CS to what ES was and the IP (offset) of SYSIN. It is jumping to a piece of relocated code in a different segment but the same offset (SYSIN).

– Michael Petch
Dec 30 '18 at 5:31





Because the value of ES is not known until runtime. One way around this is to push the segment you want to jump to, followed by the offset and then issuing a FAR RETURN. The effect is doing a runtime FAR JMP to SYSIN. It sets CS to what ES was and the IP (offset) of SYSIN. It is jumping to a piece of relocated code in a different segment but the same offset (SYSIN).

– Michael Petch
Dec 30 '18 at 5:31













Thanks Michael, now I have understood the complete logic. Thank

– Vstbutwhy
Dec 30 '18 at 6:48





Thanks Michael, now I have understood the complete logic. Thank

– Vstbutwhy
Dec 30 '18 at 6:48












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