30 replies to this topic

[Random Terrain]

Does anyone have simple definitions for the following two things:

not 'orthogonal'
So far, we have seen that what we can do with all the registers is essentially the same. Unfortunately, this is not the case with all the addressing modes! The 6502 is not 'orthogonal' - and this has some bearing on our choice of which register to use for which purpose, when designing our kernel.


non-orthogonality
That last line is interesting - an example of the non-orthogonality of our instruction set.




Thanks.


 

Edited by Random Terrain, Fri Apr 15, 2011 1:15 PM.

[ScumSoft]

Has no intersecting lines, nor right angles?

[edit] In that case then "Has both direct and indirect addressing modes"
But I can't think of an way to explain both this and what direct and indirect modes do at the same time.

Direct addressing grabs a value straight from the address.
Indirect grabs the address from the address...then the value

lda Number     = Number is a memory address which holds (5)
lda Number,Y   = Address of Number+Y (Y = 0-255) and returns value stored at Number+Y
lda (Number)   = Number holds an address not a direct value, so it returns an address which points to the value.
lda (Number),Y = Same as above but returns (Address) then Number+Y

Hmmm, going from memory here so I might have gotten this wrong

Edited by ScumSoft, Fri Apr 15, 2011 1:48 PM.

[Random Terrain]

ScumSoft, on Fri Apr 15, 2011 1:06 PM, said:

Has no intersecting lines, nor right angles?

Oops, I just added the context in my post so the meaning would be clear.

[Ed Fries]

It's a fancy word you can easily do without. All he is trying to say is that the registers aren't all equal. Some operations require certain registers. For example you can only do math with the A register. If you want to set the stack pointer you have to use the X register (TXS instruction. There is no TYS or TAS.) Etc. There are many examples.

I like to use this: 6502 opcodes to figure out what I can and can't do.

[ScumSoft]

Oh, that makes sense.
Thought it was referring to addressing modes for data. Learn something new every day

[Random Terrain]

Ed Fries, on Fri Apr 15, 2011 1:51 PM, said:

It's a fancy word you can easily do without. All he is trying to say is that the registers aren't all equal. Some operations require certain registers. For example you can only do math with the A register. If you want to set the stack pointer you have to use the X register (TXS instruction. There is no TYS or TAS.) Etc. There are many examples.

I like to use this: 6502 opcodes to figure out what I can and can't do.

Thanks. This is the exact kind of info I'm looking for.