ORA: A Special Use in Branch Testing

By Kodiak

In a recent post, I talked about some special uses for the EOR instruction, particularly with regard to saving CPU time on addition and subtraction in 6502 assembly coding on the Commodore 64.

Now it's the turn of ORA, which can also be used for adding within certain conditions on 6502, as per this Codebase piece on Combining Bits / Substitute Logical Operations.

Changing track from maths, a nice little hack you can apply (and which is used occasionally in Parallaxian) is where you start with something prosaic like this:

Bloaty way to test multiple conditions for same trigger condition
(20* cycles until EFFECT is reached, takes 16 bytes of RAM)

	LDA ZPPLANEBLOWMODE	; Disable effect if plane is exploding
	BNE QUIT
	LDA ZPTAILSLIDE	; Disable effect if plane is tail-sliding
	BNE QUIT
	LDA ZPTURNSTATUS	; Disable effect if plane is turning
	BNE QUIT
	LDA ZPRESPAWN	; Disable effect if plane is respawning
	BNE QUIT
; EFFECT	...
	...
	...
QUIT	RTS

The above assumes, in all instances of the variables being tested, that 01 = relevant condition is active and 00 = relevant condition is inactive (i.e. turned off).

Note that if any single one - or more - or all - of those conditions is / are active, the effect will not be performed; in other words, all of those conditions must = 00 for the effect to be executed.

You could, in plain English, say this:

"If any one or more of ZPPLANEBLOWMODE or ZPTAILSLIDE or ZPTURNSTATUS or ZPRESPAWN is turned on, then we do not perform the effect."

So we can convert that sentence into code thus:

Compact way to test multiple conditions for same trigger condition
(14* cycles until EFFECT is reached, takes 10 bytes of RAM)

	LDA ZPPLANEBLOWMODE	; Disable effect if plane is exploding
	ORA ZPTAILSLIDE	; Disable effect if plane is tail-sliding
	ORA ZPTURNSTATUS	; Disable effect if plane is turning
	ORA ZPRESPAWN	; Disable effect if plane is respawning
	BNE QUIT
; EFFECT	...
	...
	...
QUIT	RTS

By the same token, you could use AND everywhere instead of ORA and finish with a BEQ to the exit location, but the key aim is always the same: to save some needless branch-testing cycles and RAM by using ORA or AND in this fashion.

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Similar post:

Illegal Opcode SAX/AXS: A Practical Use

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