Bug in DosBox involving mov opcode \ VOGONS

Bug in DosBox involving mov opcode

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First post, by videogamer555

Posted on 2017-01-05, 10:36

videogamer555 Offline

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The code segment register modifier 0x8EC8 (which is the result of assembling the asm command "mov cs,ax") is supposed to transfer the 16bit value stored in the ax register into the cs register, but it's not working. In the the Dosbox Debugger, the debugging console is showing that it's producing the error "Unhandled opcode 8EC8". I believe this is a bug in DosBox, as it's supposed to be able to handle the entire x86 instruction set.

Reply 1 of 5, by Qbix

Posted on 2017-01-05, 10:39

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mov cs,ax is not an allowed opcode/operation.

Last edited by Qbix on 2017-01-05, 12:16. Edited 1 time in total.

Reason: remove unneeded link

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Reply 2 of 5, by ripsaw8080

Posted on 2017-01-05, 11:36

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MOV CS,AX only works on an 8088, not on later processors, and DOSBox doesn't emulate 8088-specific behavior. The same is true of the POP CS instruction, where the 0Fh encoding became a prefix for a group of instructions on later processors.

Reply 3 of 5, by videogamer555

Posted on 2017-01-05, 23:21

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Qbix wrote:
mov cs,ax is not an allowed opcode/operation.

Ok. That makes sense. How do you modify the cs register then?

Reply 4 of 5, by Harekiet

Posted on 2017-01-06, 00:03

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A far jump/call/return

Reply 5 of 5, by vladstamate

Posted on 2017-01-06, 18:28

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Something like this would work

MOV AX, your_new_cs_value
PUSH AX
MOV AX, offset_to_your_new_code
PUSH AX
RETF

The reason stuff like MOV CS, AX or POP CS were cut in later processor is because they are really broken since the prefetch queue still has data fetched from the old CS and it will end up with partially decoded instructions and it won't really behave (even on 8088/86) the way you would like to.

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