6502 boundary question

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Anes
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6502 boundary question

Post by Anes » Tue Jun 21, 2005 11:44 pm

Im little confused about what it means "page boundary crossed" when doing an Instruction that for example reads from memory, like ADC addresing mode ABSOLUTE_X, the reference says (as almost all do) "add 1 cycle if the "page boundary is crossed". Is it the fact that the base address is given by the pc + 1 and pc + 2 and added to x? i mean if we do:

lda $01FF, x

is in that situation that we cross the page boundary to $02XX, assuming that x > 0 of course.

Is that well?
ANes

Muchaserres
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Post by Muchaserres » Wed Jun 22, 2005 3:44 am

Yup, you got it.

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blargg
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Post by blargg » Wed Jun 22, 2005 9:46 am

My understanding of the reason for this extra cycle is that the 6502 has a single general-purpose 8-bit adder that it reuses for various tasks, including indexed absolute addressing mode. On the first cycle it adds X to the low byte of the target address. If a carry is generated, it needs to take an extra cycle to add one to the high byte. This also explains why the low byte of addresses is stored first.

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