Quick IN/OUT question

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score_under

Joined: 27 Aug 2009
Posts: 27

score_under 20 Jun 2010, 16:22

Is:

Code:

in al,dx
xchg al,ah
in al,dx
xchg al,ah

The same as

Code:

in ax,dx

?

And how about:

Code:

xchg al,ah
out dx,al
xchg al,ah
out dx,al

Is that the same as:

Code:

out dx,ax

?

And finally, are "rep outs[b/w/d]" interchangeable so long as the count is modified accordingly?

20 Jun 2010, 16:22

Madis731

Joined: 25 Sep 2003
Posts: 2139
Location: Estonia

Madis731 20 Jun 2010, 18:35

Usually the documentation tells you what they want - if you are told to pass WORD, then you cannot pass two BYTEs and the other way round, because what you pass (BYTE, WORD, DWORD) will change the behaviour of the IN and OUT.

20 Jun 2010, 18:35

baldr

Joined: 19 Mar 2008
Posts: 1651

baldr 20 Jun 2010, 19:28

score_under,

in al, dx reads byte-sized port, whereas in ax, dx reads word-sized one. Reading byte-sized port twice rarely substitutes for latter. The same for ins / out / outs.

Most modern hardware understands word-sized out instead of two byte-sized to adjacent ports. Good example of this is VGA programming (GC and CRTC registers follow index/data scheme).

20 Jun 2010, 19:28

DOS386

Joined: 08 Dec 2006
Posts: 1905

DOS386 27 Jun 2010, 11:05

> Is that the same as

Definitely NOT. May "seem to work" with some hardware (even worse with NTVDM "hardware") but is different and such bad use of IN or OUT may hang the PC Sad

PS: you can't read the PIC as WORD, and when reading 2 separate Byte's, the result can be wrong, guess why Sad

27 Jun 2010, 11:05

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