You should probably use better topic titles. I edited your topic name from "F" to "A couple of quick questions regarding the flags register"

You should probably use better topic titles. I edited your topic name from "F" to "A couple of quick questions regarding the flags register"

Oh, okay. I was just bored

Quote from: Deep Thought on September 21, 2010, 07:33:12 pm

A couple of quick questions regarding the flags register:

• Is H basically a carry flag for the right nibble of the accumulator? Meaning if I do
  

Code: (TI-ASM) [Select]

LD A,15
INC A

it should be set, right?[/li]
[li]Is V set when the 7th bit changes from one to zero? Meaning if I do[/li]

Code: (TI-ASM) [Select]

LD A, 128
DEC A

should it be set?[/li]

[/quote]
Mostly correct on both points. However, it should be clarified that the overflow is set if an addition/subtraction carries across the 127 to -128 mark (which it does in your example)

Edit:
Also, maybe this will make it clearer. If the result of the addition or subtraction (when you don't lock the result to 8 bits) is less than -128 or larger than 127, the overflow flag is set.
[/quote]

Okay, thanks!

The inc and dec instructions don't affect flags when used with 16-bit register pairs.

For some reason, the carry flag is NOT set when a number overflows with inc/dec, not even an 8-bit one.

For example:

ld l, 255
inc l

does not affect the carry flag.

For some reason, the carry flag is NOT set when a number overflows with inc/dec, not even an 8-bit one.

For example:

ld l, 255
inc l

does not affect the carry flag.

That got me good when I first started learning Z80 ASM. For a quick reference, take a look at this thing that every Z80 ASM developer ought to have: http://www.ticalc.org/pub/text/z80/z80instr.txt

I frequently view it when I'm unsure what flags any given instruction will affect. A "." indicates that the flag is not affected and a "*" indicates that the flag will be affected by its definition, whatever that may be (like when you add two numbers together and the result is over 255, the carry flag will be set). It is rather convenient how they set it all up, too.

EDIT: More often than not, I'll view that file just to find out how many bytes that particular instruction will take up. Great for those labelless jumps like "jr $+N"

It's "Len" or "Length". That's the number of bytes that instruction takes.
EDIT: That's what I really use that file for

Oh. Those are machine cycles. Since the shortest instruction is 4 T-cycles (I like to use the term "clock cycle" or CC for this), it might be easier to represent some instructions in terms of machine cycles. The only real benefit is dealing with smaller numbers, though I don't really like using them because of all those odd instructions that will not divide evenly with 4. Like your 2 byte ADD instructions (11CCs) and your two byte inc/dec instructions (6CCs).