ROM calls and other tricks

[J.E.E.K.]

Remove NOP’s
[...]

This list could be extended by

• Hold flag in bit 7 of a location, so you can check the flag by "BIT flag" followed by "BMI/BPL destination" keeping A untouched, clear the flag with "LSR flag". If carry is set by an condition before, use "ROR flag" to set the flag.
• Operations without temporarily save the accumulator to memory:
  • To subtract the value in A from location's value ( (mem)-A ) use EOR #$FF; SEC; ADC mem (negate A with two's complement and simply add).
  • Merge some bits from A into a memory location using EOR mem; AND #mask_bits_taken_from_acc; EOR mem; STA mem.

[bjonte]

Sometimes you want to toggle a memory location between two values. This can be done using EOR. The value to use for EOR can be constructed with eor in the assembler (operator ^ in this example).

Code: Select all

// constants to toggle between
const value1 = 55
const value2 = 170

// set initial value
lda #value1
sta memaddr

// toggle
lda memaddr
eor #value1 ^ value2
sta memaddr

[MrSterlingBS]

Sometimes I used

STY $ZP
… some code
LDY $ZP

instead of
TYA
PHA
… some code
PLA
TAY

A little bit faster and shorter code if you have some ZP registers free.

[wimoos]

Sometimes you want to toggle a memory location between two values. This can be done using EOR. The value to use for EOR can be constructed with eor in the assembler (operator ^ in this example)

To elaborate on this, consider the following code snippet that I used in WimBasic:

Code: Select all

	EOR  #$82      	 ; token NEXT ?
	BEQ  LA3D6     	 
	EOR  #$9C ^ $82  ; token CLR
	BEQ  LA3F3     	 
	EOR  #$8E ^ $9C  ; token RETURN
	BEQ  LA46C     	 
	JMP  $CF08 ; ?SYNTAX ERROR
;
LA3D6	TAY ; clear Y
		...
		
LA46C	

The code is entered with a tokenized value in A, that needs to be checked against three possible values. The code, following the selection benefits from having a value of zero in A (or could benefit from the carry-flag not having been changed).

Regards,

Wim.