Category:6502 Assembly: Difference between revisions
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The 6502 has much fewer registers than its contemporaries, and as such the zero page is useful as a set of "registers," albeit slower. The 6502 is also limited in its stack operations, as it cannot push X or Y onto the stack directly, and must destroy the accumulator in order to do so. This creates a problem when a function needs to preserve multiple registers yet takes its input from the accumulator. The easiest solution is to use a zero page memory address to preserve the accumulator and the stack for X and Y. (Or vice versa.)
On the 65816, the zero page is called the "direct page," and it can be relocated. The 65816's D register points to the direct page. The location of the direct page can be changed at runtime using the <code>TCD</code> command. This feature lets the programmer set up different pages of RAM for different tasks, and switch the direct page to that page temporarily to speed up that task. Unfortunately, this also makes it very difficult to read someone else's assembly and figure out what they're actually doing, as it's not clear what memory addresses they're actually loading from.
==Little-Endian==
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