writing more than byte to port

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vid
Verbosity in development

Joined: 05 Sep 2003
Posts: 7105
Location: Slovakia

vid 19 Jul 2007, 21:53

hi, i am confused about writing more than one byte to port.

Does this code:

Code:

mov dx, 1234
out dx, ax

write one byte to 1234 and other to 1235, or does it write both to 1234, or is it port-dependent?

19 Jul 2007, 21:53

Pavia

Joined: 20 Jul 2007
Posts: 2
Location: Russia

Pavia 20 Jul 2007, 01:28

Is undoc.
IMHO
For all ports. Code OUT DX,AX write AL in DX and AH in DX+1.
Execpt only HDD Data Register

20 Jul 2007, 01:28

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20309
Location: In your JS exploiting you and your system

revolution 20 Jul 2007, 04:47

The I/O system is similar to the memory system as far as where the bytes go.

You can write OUT dx,ax and two bytes are sent the port, the lsb to dx, and msb to dx+1. The same for a 32bit write, lsb to dx and msb to dx+3.

If you have an older machine (ISA bus) then some hardware cards did not support 16 bit writes so you also need to be mindful of what device you are writing to.

20 Jul 2007, 04:47

MazeGen

Joined: 06 Oct 2003
Posts: 977
Location: Czechoslovakia

MazeGen 20 Jul 2007, 07:23

Intel Basic Architecture Manual wrote:

Any two consecutive 8-bit ports can be treated as a 16-bit port, and any four consecutive
ports can be a 32-bit port. In this manner, the processor can transfer 8, 16, or
32 bits to or from a device in the I/O address space.

Intel Instruction Set Manual wrote:

The size of the I/O port being accessed is determined by the opcode for an 8-bit I/O
port or by the operand-size attribute of the instruction for a 16- or 32-bit I/O port.

Code:

OUT DX, AL ; 8-bit port
OUT DX, AX ; 16-bit port
OUT DX, EAX ; 32-bit port

Note that there is no OUT DX, RAX instruction.

20 Jul 2007, 07:23

vid
Verbosity in development

Joined: 05 Sep 2003
Posts: 7105
Location: Slovakia

vid 20 Jul 2007, 09:30

thanks...

20 Jul 2007, 09:30

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