In my last post I showed how the Action! compiler produces some pretty optimized code for CARD math under certain circumstances. This time I will show the more general case which should be pretty familiar to anyone who has done 6502 programming.

Here is the Action! program and it’s dis-assembly:

CARD I

PROC MAIN()
I=2
I=I+2
RETURN

0E6C: .BYTE 00,00
0E6E: 4C 71 0E    JMP  $0E71      

;I=2
0E71: A0 00       LDY  #$00     
0E73: 8C 6D 0E    STY  $0E6D      
0E76: A9 02       LDA  #$02     
0E78: 8D 6C 0E    STA  $0E6C    

;I=I+2  
0E7B: 18          CLC        
0E7C: AD 6C 0E    LDA  $0E6C      
0E7F: 69 02       ADC  #$02     
0E81: 8D 6C 0E    STA  $0E6C      
0E84: AD 6D 0E    LDA  $0E6D      
0E87: 69 00       ADC  #$00     
0E89: 8D 6D 0E    STA  $0E6D      
0E8C: 60          RTS         

Most of what is here we have discussed before so I won’t go into great detail. As you can see since I is initialized to the value 2 the INY optimization can’t be used so two loads and stores are performed. It’s interesting to note that a different register is used for each byte. I am not sure why the compiler chooses to do this, although in the end it doesn’t affect program size or performance.

The add part of the program is standard 6502 16-bit math, adding the lower byte, then adding the upper byte which also handles and carry from the lower byte.