help with the Replacement for XLAT

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MAD_DËMON

Joined: 03 Mar 2006
Posts: 23

MAD_DËMON 20 May 2006, 17:59

The Last rev of AMD64 Volume1: App programming says:

Quote:

􀂄 XLAT—Translate Table Index
The XLAT instruction replaces the value stored in the AL
register with a table element. The initial value in AL serves as
an unsigned index into the table, and the start (base) of table is
specified by the DS:rBX registers (depending on the effective
address size).
This instruction is not recommended. The following instruction
serves to replace it:
MOV AL,[rBX + AL]

I try both "mov al,[bx+al]" and "mov [rbx+al]" but FASM gives me a syntax error

anyone can say me what is wrong?

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20 May 2006, 17:59

r22

Joined: 27 Dec 2004
Posts: 805

r22 20 May 2006, 18:24

32bit XLAT
mov al, byte[ebx+eax]
64bit XLAT
mov al, byte[rbx+rax]

The problem with mov al,[rbx+al] is AL's a 8bit register so it doesn't match the memory addressing.

If you are ONLY using the AL part of RAX/EAX then you can use MOVZX instead of MOV. Using the ZX suffix is a sort of optimization for 64bit systems in much the same way is XOR reg,samereg is an optimization for zeroing the register.

20 May 2006, 18:24

MAD_DËMON

Joined: 03 Mar 2006
Posts: 23

MAD_DËMON 20 May 2006, 18:39

well then it seems that the AMD documentation was wrong this time.. Sad

20 May 2006, 18:39

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