OFF TOPIC! General Registers

[MikeB]

I am reviewing some assembly notes, simply out of curiosity, and I come across something that I am not sure of. In the 8088 processor, you can access the general registers high byte and low byte by denoting with a H or L respectively. In DEBUG, I typed the following:

- R AH

and it gives me an error. Can I not access the high and low byte of the general registers on a P4 processor?

Mike B

[Yves M]

You definitely can. But what is that "R" instruction ? try something like "ADD AL, BL", that should work correctly.

[cup]

r cannot be used to modify the 8-bit registers. The best you can do is look at the AX value and modify the upper 8 bits.

debug.exe is a 32-bit version of a 16-bit program. It is a hangover from the DOS days. The r instruction for modifying registers only seems to work for the 16-bit registers. .

I use it in batch files as a version of Unix's sed. It is absolutely brilliant for that.

[MikeB]

You definitely can. But what is that "R" instruction ? try something like "ADD AL, BL", that should work correctly.

I am looking at it using DEBUG.EXE where R instruction is used to edit a register.

r cannot be used to modify the 8-bit registers. The best you can do is look at the AX value and modify the upper 8 bits.

So what can I use to set the value in the AL and HL?

Nothing?

Mike B

[MikeB]

Sorry I figured it out. You cannot directly access the H and L other then actually specifying the values in AX. So:

- R AX
AX 0000
: 0102

-R IP
IP 0100
: 100

- E 100 
0AEA:0100 AA.00 43.C4

The above code will add the high and low bytes of AX
ADD AH, AL

Mike B

[cup]

Like I said earlier, you have to use AX to set it. eg if you wish to set AL to 99 hex

r AX
AX 1234
:1299

Similarly, if you wish to set AH to 50 hex

r AX
AX 1234
:5034