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Program Counter: Difference between revisions
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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. | 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. | If incremented past FFFFh, it wraps around to zero.<sup>[E&L, page 34]</sup> | ||
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. | 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. | ||
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* [[BRA]] | * [[BRA]] | ||
* [[JMP]] | * [[JMP]] | ||
* [[RTS]] | |||
* [[RTL]] | |||
=== References === | === References === | ||
Latest revision as of 20:42, 10 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.[E&L, page 34]
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.
See Also
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