Re: Page Boundary Question

[mjmahon@aol.com (Michael J. Mahon)]

Bryan Parkoff asked:

>    I understand that 6502 can only add one extra cycle if it is across page
>boundaries.
>
>    For example,
>
>2000: A9 00    LDA #00
>2002: F0 EC    BEQ 1FF0
>
>    If BEQ is true, it will branch to $1FF0.  Extra cycle is supposed to be
>added, but it didn't.
>
>2080: A9 00    LDA #00
>2082: F0 7F    BEQ 2103
>
>    If BEQ is true, it will branch to $2103.  Extra cycle will be added.
>($20)$82 + $7F will always show carry flag that will let cycle to know
>before it is added.  How can 6502 know that ($20)$04 + $EC will not always
>show carry flag?  Please explain.  It is what 6502 Reference Manual says
>that branch can only add one extra cycle if carry flag is present.

It's easy--it just does the signed addition and tests whether there
was a carry out.  If there was, then the high-order address byte
must be incremented/decremented by one, and that operation
takes the extra cycle.

None of this affects the programmer-visible carry flag.

-michael

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