Index, Shadow, and Other Registers

z80 » Advanced

So far you only know af, bc, de, and hl as registers. Sometimes you probably run out of registers to use. There are several others you can work with too but they have their limits.

Index Registers

Index Registers are register pairs and cannot be broken up to use as i, y, or x. The ix register is free to use at any time. It is used synonymously with hl. The assembler just puts an extra byte in to tell the z80 that it is going to use ix in the next instruction instead of hl. This slows down the speed a little and adds an extra byte so don't use it unless you have to. The iy is usually holding the value for the beginning of a table for the System Flags for TI-OS. Say you wanted to write inverse video. You would type the following:
set textflags,(IY+textInverse)
res textflags,(IY+textInverse)

The first command sets the text to white on black and the second line resets it to the normal black on white. You can use iy and ix to make your own tables and arrays.

Shadow Registers

Shadow Registers are a whole other set of all the registers. It's like pushing and popping all the registers. This is useful and faster when you have a routine that destroys all the registers and you want to get them back later. Instead of pushing and popping them all, you just switch them with the shadow registers. You must disable interrupts by 'di' before using these. Type this to switch out the registers:
ex af,af'	;switch af with it's shadow
exx		;switch bc,de, and hl with their shadows
Do the same thing when you're done using them to get the original register values back.

Other Registers

Other Registers you can use are the i and r registers. The i register is used with interrupts. It just holds the higher byte of the address that should be jumped to when an Interrupt occurs. The r register is called the Memory Refresh Register. It is incremented after each instruction (sometimes twice) executed by the processor. Bit 7 is never set though, so it has a maximum value of 127, I don't know why.

It is useful for low-quality random number routines but if you use it a lot in a row, your numbers won't be so random because they will use, for example, 1 and then use 2 and then use 3 and so on. Here's an OK random number generator you can use that will have the above problem if used several times in a row. It will return a number between 1 and 16.

	ld a,r
	and %00001111	;is now between 0 and 15
	inc a			;is now between 1 and 16

More from z80 » Advanced
All The Ports // APD // Assembler Directives // Entry Stack // User Fonts // IM 1 // IM 2 // Index, Shadow, and Other Registers // User Interrupt // Morphic Code // On-Off // Reading Keypresses from Port // Shift and Rotate // Simulating Key Presses // Sound // Square-Root Programs // System Flags of TI-OS