Parameter Passing: Difference between revisions
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→Example 6502 Assembly
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LDX $20 ;load the value at memory address $20 into the X register.
DEX ;subtract 1 from the value in the X register. This is only a copy. The actual value stored at $20 is unchanged.
;Keep in mind that this is true for INX/INY/DEX/DEY but not INC/DEC.
BIT $2002 ;Set the flags as if the value stored at address $2002 was bitwise ANDed with the accumulator.
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LDA $40 ;load the value stored at memory address $40 into the accumulator
ROR A ;rotate right the accumulator. This is only a copy. The actual value stored at $40 is unchanged.</lang>
The main takeaway from this is that by default, the use of <code>LDA/LDX/LDY</code> commands to load from memory is pass-by-value. In order to actually update the memory location, you must store the value back into the source memory address.
====Functions====
Whether a function's parameters are pass-by-value or pass-by-reference ultimately depends on how it is written and which opcodes are used.
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