Opcode .. An original has does not correctly fetch the target address if the indirect vector falls on a page boundary (e.g. $xxFF where xx. Instruction set of the MOS // MPU. Notably, there are no legal opcodes defined where c = 3, accounting for the empty columns in the usual. Shown below are the instructions of the , 65C02, and 65C processors. GREEN . 10 instructions. These have a completely different set of opcodes.
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Unofficial opcodessometimes called illegal opcodds or undocumented opcodesare CPU instructions that were officially left unused by the original design. The family datasheet from MOS Technology does not specify or document their function, but they actually do perform various operations.
Some of these instructions are useful; some are not predictable; some do nothing but burn cycles; some halt the CPU until reset.
Most NMOS cores interpret them the same way, although there are slight differences with the less stable instructions. An accurate NES emulator must implement all instructions, not just the official ones. A small number of games use them see below. The microcode of the is compressed into a entry decode ROM. Instead of entries telling how to process each separate opcode, it’s encoded as combinational logic post-processing the output of a “sparse” ROM that acts in some ways like a programmable logic array PLA.
Each entry in the ROM means “if these bits are on, and these bits are off, do things on these six cycles. Often this is on purpose, such as one line for the addressing mode and one for the opcode part. But many of the unofficial opcodes simultaneously trigger parts of the ROM that were intended for completely unrelated instructions.
Perhaps the pattern is easier to see by shuffling the ‘s opcode matrix. This table lists all opcodes, 32 columns per row. The columns are colored by bits 1 and 0: But if we rearrange it so that columns with the same bits are close together, correlations become easier to see:. Thus the 00 red block is mostly control instructions, 01 green is ALU operations, and 10 650 is read-modify-write RMW operations and data movement instructions involving X.
The Instruction Set Decoded
This causes instructions to have strange mixing properties. Some even differ based on analog effects.
The use of unofficial opcodes is rare in NES games. It appears to occur mostly in late or unlicensed titles:. CPU unofficial opcodes From Nesdev wiki. The following documents may provide reference for these instructions: Retrieved from ” https: Personal tools Create opdodes Log in.