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Index > Compiler Internals > Instruction too optimized & lea segment instruction

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CandyMan



Joined: 04 Sep 2009
Posts: 322
Location: film "CandyMan" directed through Bernard Rose OR Candy Shop
CandyMan
1. Instruction too optimized
Code:
jmp  word $+16 ; nasm: E9 0D 00, fasm: EB 0E    


2. maybe ignoring segment override in lea instruction?
Code:
lea ax,[es:di]    

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smaller is better
Post 13 Nov 2020, 17:56
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Tomasz Grysztar



Joined: 16 Jun 2003
Posts: 7802
Location: Kraków, Poland
Tomasz Grysztar
1. The semantics used by fasm focus on what the instruction does, not how it is encoded. The size override does not reflect the size of immediate field in the instruction, it corresponds to an actual size of the target address:
Code:
jmp word -1     ; EB FD         jumps to 0FFFFh
jmp dword -1    ; 66 EB FA      jumps to 0FFFFFFFFh    


2. Yes, the segment prefixes could be safely ignored there, I guess this never really came up, as you rarely see LEA instruction used this way.
Post 13 Nov 2020, 18:02
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CandyMan



Joined: 04 Sep 2009
Posts: 322
Location: film "CandyMan" directed through Bernard Rose OR Candy Shop
CandyMan
I understand.

Thanks for the quick reply.
Post 13 Nov 2020, 18:42
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