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Load IWA with Integer from Zero Page Address

Submitted by Steve Fewell

Routine: izpin
Name: Load IWA with Integer from Zero Page Address
Starting Address: &AA80
Entry criteria: X is the Zero page address of the Least Significant (first) byte of a 32-bit Integer variable
Exit: IWA contains the Integer number pointed to by X (That is the 32-bit Integer stored in locations 00,X to 03,X).

Description:
Loads the IWA with the 32-bit Integer pointed to by the X register (The number is stored least significant byte first, one byte is copied at a time), and sets A to #&40 to show that an Integer number is being processed.

There is another routine which loads the IWA from any address, but we need this zero page routine due to it executing a lot faster than the one which allows longer addresses.

Disassembly for the Load Integer from Zero Page Address routine

AA80   181 000 B5 00 LDA &00,X
AA82 * 133 042 85 2A STA &2A
AA84   181 001 B5 01 LDA &01,X
AA86 + 133 043 85 2B STA &2B
AA88   181 002 B5 02 LDA &02,X
AA8A , 133 044 85 2C STA &2C
AA8C   181 003 B5 03 LDA &03,X
AA8E - 133 045 85 2D STA &2D
AA90 @ 169 064 A9 40 LDA#&40
AA92 ` 096 60 RTS

 


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