How To Conveter A 8/16(fx ah/dx) to a 32(fx esi)

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gumletis

Joined: 18 Dec 2004
Posts: 128

gumletis 21 Jan 2005, 08:55

Hi, im working with both, microsoft system and dos, i wanna know how to conveter a 8/16 bit register to a 32 bit... how??? please help
thanks - best regards

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21 Jan 2005, 08:55

beppe85

Joined: 23 Oct 2004
Posts: 181

beppe85 21 Jan 2005, 10:33

Depends if the number is signed or not.

Code:

Signed:
movsx eax, al ; move sign-extended

Unsigned:
movzx eax, al ; move zero-extended

Note that both source and dest registers can be (part of) the same.

21 Jan 2005, 10:33

MCD

Joined: 21 Aug 2004
Posts: 602
Location: Germany

MCD 24 Jan 2005, 16:55

for signed registers, you should better use

Code:

cbw
cwde

since movsx always fills the operand up with "1"s, ignoring if source operand was actually positive or negative.

Note that these instructions only work for the accumulators. You will have use additional move instructions if you want the result in other registers.

24 Jan 2005, 16:55

vid
Verbosity in development

Joined: 05 Sep 2003
Posts: 7105
Location: Slovakia

vid 24 Jan 2005, 22:58

Quote:

since movsx always fills the operand up with "1"s, ignoring if source operand was actually positive or negative.

really?!? i can't believe that. Too bad i can't try now

24 Jan 2005, 22:58

mike.dld

Joined: 03 Oct 2003
Posts: 235
Location: Belarus, Minsk

mike.dld 24 Jan 2005, 23:14

Calm down, man Wink

movsx fills higher part with source operand' sign bit
See Intel's manual "Volume 1: Basic architecture", Chapter 7.2.1.4, Figure 7-6

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24 Jan 2005, 23:14

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