Category:6502 Assembly: Difference between revisions
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==A True 8-Bit Computer== |
==A True 8-Bit Computer== |
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The 6502 is an 8-bit computer in the purest sense. Unlike the Z80, the 6502 is not capable of 16 bit operations within a single register. To work with a 16 bit number you will need to split it in two and work with each half individually. The carry flag is useful for this, as (like on other CPUs with a carry flag) it acts as a conditional addition |
The 6502 is an 8-bit computer in the purest sense. Unlike the Z80, the 6502 is not capable of 16 bit operations within a single register. To work with a 16 bit number you will need to split it in two and work with each half individually. The carry flag is useful for this, as (like on other CPUs with a carry flag) it acts as a conditional addition, as in the example below. |
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<lang C>unsigned short foo = 0x00C0; |
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foo = foo + 0x50;</lang> |
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Equivalent 6502 Assembly: |
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<lang 6502asm>LDA #$C0 |
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STA $20 ;we'll use $20 as the memory location of foo, just to keep things simple. A real C compiler would use the stack. |
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LDA #$00 |
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STA $21 ;low byte was #$C0, high byte was #$00 |
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;now we add #$50 |
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LDA $20 ;load #$C0 |
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CLC |
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ADC #$50 |
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STA $20 |
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LDA $21 |
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;this time we DON'T clear the carry before adding. |
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ADC #0 ;since there's a carry from the last addition, this actually adds 1! If there was no carry, it would add 0. |
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STA $21</lang> |
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==Processor Flags== |
==Processor Flags== |
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