All the Flags

z80 » Intermediate

You already know all the flags that are important and used the most but here is the full list. Remember these are all bits in the f register and only six of them are used.

Symbol Flag Name Bit
S sign 7
Z zero 6
unused unused 5
Ac Auxiliary Carry 4
unused unused 3
P Parity Even or Odd 2
N add/subtract 1
C carry 0

The only flags that really matter here have already been discussed in the Flags and Conditions Section so you don't have to really worry. There is, however, one flag here that you might want to know about: the parity flag. The instruction cpir can be used with this.

This instruction is called Block Search instruction. It is used to search starting at hl's address for bc amount of bytes after until the byte at address hl matches the byte in a. It keeps on searching (and decreasing bc every time it increases hl) until it either finds a match or bc is down to zero. You can't really tell which of those two happened without checking the status of bc.

Here's some code that will start looking for the value 69 in a 50 byte table starting at the label numbers. If it doesn't find 69 in the table before bc runs out, it'll jump to the routine not_found; else, it'll continue.

	ld hl,numbers
	ld bc,50
	ld a,69
	jp po,not_found

Cpir has a twin instruction named cpdr. The only difference between these two instructions is that cpdr decreases the address hl each iteration whereas cpir increases it.

More from z80 » Intermediate
All the Flags // Debugging // Down-Left Bug // _GetKey Codes // Logical Operators // Memory, ROM, RAM, and Safe Areas // Miscellaneous Instructions // PC and SP // Random Numbers // TI's ROM Calls // Restart Commands // Simulated 16-bit Addition // The Stack // Tables and Arrays // Text Display // Variables