Program Counter: Difference between revisions

Revision as of 04:19, 9 August 2024

The Program Counter (PC) keeps track of which instruction is currently executing. On both the 65c816 and S-SMP it is 16 bits wide. The low byte is called PCL and the high byte is called PCH.

If incremented past FFFFh, it wraps around to zero.

The 6502 had 16-bit absolute addressing but only an 8-bit adder, so in emulation mode branches that cross a page boundary incur a one cycle penalty. 65c816 native mode has no such penalty because the full 16-bit adder is used.

References

Eyes & Lichty, page 33
subparagraph 8.1.4 on page 3-8-4 of Book I of the official Super Nintendo development manual
Pickens, John. http://www.6502.org/tutorials/6502opcodes.html#PC
Clark, Bruce. http://www.6502.org/tutorials/65c816opcodes.html#6.2.1.1

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-The '''Program Counter''' (PC) keeps track of which instruction is currently executing.  If incremented past FFFFh, it wraps around to zero.
+The '''Program Counter''' (PC) keeps track of which instruction is currently executing.  On both the [[65c816]] and [[S-SMP]] it is 16 bits wide.  The low byte is called PCL and the high byte is called PCH.
+If incremented past FFFFh, it wraps around to zero.
 The 6502 had 16-bit absolute addressing but only an 8-bit adder, so in [[emulation mode]] branches that cross a page boundary incur a one cycle penalty.  65c816 native mode has no such penalty because the full 16-bit adder is used.
-On the [[S-SMP]] it is 16 bits wide.  The low byte is called PCL and the high byte is called PCH.
 === See Also ===