Page Crossing on Compare Absolute X

[Toyman]

Hi

How does the Compare Absolute (DD) determine Page Crossing ?

Code: Select all

FD07  CMP $8003,X

Where
Accumulator = $30
X Register = $05
Thus the Compared address will point to $8008 which = 0.

I've got a 6502 Simulator that adds an extra cycle because of a Page Crossing. Is this correct ? And if so why ?

Thanks

[8BIT]

Toyman wrote:

Hi

How does the Compare Absolute (DD) determine Page Crossing ?

Code: Select all

FD07  CMP $8003,X

Where
Accumulator = $30
X Register = $05
Thus the Compared address will point to $8008 which = 0.

I've got a 6502 Simulator that adds an extra cycle because of a Page Crossing. Is this correct ? And if so why ?

Thanks

As written above, your instruction should use 4 clock cycles because no page boundry was crossed. However, if it is coded like this:

Code: Select all

FD07  CMP $80FD,X

where X=$05, then a page bountry is crossed (compare address is $8102).
In this case, an additional cycle is used (5 total). The extra cycle is needed to increment the upper word of the target address (from $80 to $81).

Hope that helps!
Daryl