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Tomasz Grysztar 10 May 2004, 19:12
The way expression is calculated depends on the predicted output size, because if you have done some logical operations on the small values extended to 64-bit and then wanted to fit it into small value, it might happen that value would no fit in smaller amount of bits. For example "not 8Fh" performed on 64-bits is the same as "-90h" value, and so "db not 9Fh" would cause "value out of range" error. But fasm knows that argument to the "db" directive has to be 8-bit value, so when it calculates "not", does it only on the low 8 bits.
Every time an expression calculator is used, it needs to have specified the size of the output value it has to make, so it can adjust those operations appropriatelly. And so for "mov eax,not 8Fh" is does operation on the 32 bits, and for "mov al,not 8Fh" on the 8 bits. In case of numerical constant definition, default size is the maximum (64 bits), so this code: Code: const = not 8Fh db const will fail, the constant has 64-bit value of -90h, which cannot fit in the byte. But you can adjust the size of value you want to get with the size operator: Code: const = byte not 8Fh db const in this case the logical "not" was performed on 8 bits only, and constant got value 70h this way. How it was adapted for the intended "bswap" behavior, it is simple, I think. |
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10 May 2004, 19:12 |
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vid 11 May 2004, 06:56
so, i thought size operator in expressions is just truncation, now i see it has more special role.
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11 May 2004, 06:56 |
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comrade 08 Aug 2004, 00:45
Was this feature ever officially implemented? My memory was sparked by http://board.flatassembler.net/topic.php?t=2005
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08 Aug 2004, 00:45 |
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Matrix 11 Sep 2004, 10:22
Code: bswapw: xchg al,ah ret bswapd: ; 01 23 45 67 xchg al,ah ; 01 23 67 45 rol eax,16 ; 45 67 01 23 xchg al,ah ; 45 67 23 01 ret This is very easy MATRIX |
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11 Sep 2004, 10:22 |
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Madis731 11 Sep 2004, 11:05
In 32bit mode it is very slow and also xchg is very clumsy.
Code: bswapd: ; 01234567h mov ebx,eax ; 01234567h rol eax,8 ; 23456701h ror ebx,8 ; 67012345h and eax,000FF00FFh ; 00450001h and ebx 0FF00FF00h ; 67002300h or eax,ebx ; 67452301h ret much faster, at least in my tests 
-BUT- uses two registers |
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11 Sep 2004, 11:05 |
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Matrix 11 Sep 2004, 13:21
Madis731 wrote: In 32bit mode it is very slow and also xchg is very clumsy. 
yes, my poing was exactly to emulate the cpu function withoust using memory or other registers but its nice code you have there MATRIX |
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11 Sep 2004, 13:21 |
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S.T.A.S. 11 Sep 2004, 19:54
Yes, bswap is slow on Pentium IV (very *advanced* CPU :/) - this is documented by intel.
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11 Sep 2004, 19:54 |
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comrade 12 Sep 2004, 00:10
what about the directive itself? is it in fasm?
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12 Sep 2004, 00:10 |
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Matrix 15 Sep 2004, 18:26
Madis731 wrote: In 32bit mode it is very slow and also xchg is very clumsy. Hy again, that is might be because newer CPU's can do more of the primitive operations in one tick, that's nice, getting closer to RISC architecture the bad news is there are some instructions can't be run together, and there can be many trouble making some code faster. there are some C compilers which make this for you - for example new microcontrollers are RISC. But these compilers are at a very high cost ! MATRIX |
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15 Sep 2004, 18:26 |
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