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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If the zero flag is clear then add the relative displacement to the program counter to cause a branch to a new location. Finally, a more complex view, the instruction set listed by rows as combinations of a and c, and b in columns: CPU unofficial opcodes From Nesdev wiki.

The Instruction Set Decoded

JSR is the only absolute-addressing instruction that doesn’t fit the aaabbbcc pattern. This instructions is used to test if one or more bits are set in a target memory location.

Perhaps the pattern is easier to see by shuffling the ‘s opcode matrix. However, these alternate NOPs are not created olcodes. These have a completely different set of opcodes:. The aaa and cc bits determine the opcode, and the bbb bits determine the addressing mode.

It appears to occur mostly in late or unlicensed titles:. The use of unofficial opcodes is rare in NES games.

The bit manipulation instructions found only on the Rockwell and WDC versions of the 65C02 are not included in the table, nor are the “undocumented” instructions of the original But apparently this isn’t always reliable–there are reports of some opocdes these instructions occasionally locking up the processor.


If the carry flag is clear then add the relative displacement to the program counter to cause a branch to a new location. Similarly, the test-and-branch instructions pocodes of the form xyyywhere x is 0 to test whether the bit is 0, or 1 to test whether it is 1, and yyy is which bit to test.

Opcodes – Plus/4 Encyclopedia – Plus/4 World

In both cases you should include an explicit CLD to ensure that the flag is cleared before performing addition or subtraction. Some have one- or two-byte operands which they don’t do anything withand they take different amounts of time to execute.

The bit set and clear instructions have the form xyyywhere x is 0 to clear a bit or 1 to set it, and yyy is which bit at the memory location to set or clear. If the zero flag is set then add the relative displacement to the program counter to cause a branch to a new location.

Bits 7 and 6 of the value from memory are copied into the N and V flags. In some cases the 01 and 10 instructions are incompatible.

An increment with carry may affect the hi-byte and may thus result in a crossing of page boundaries, adding an extra cycle to the execution. Usually some mixture of the two, in a manner that varies depending on who opfodes thewhen it was made, the phase of the moon, and other unpredictable variables.

6502 Instruction Set

Most instructions that explicitly reference memory locations have bit patterns of the form aaabbbcc. The bit that was in bit 0 is shifted into the carry flag. If overflow occurs the carry bit is set, this enables multiple byte addition to be performed. A memory read does occur, generally using the addressing mode you would expect from the bit patterns–which may be significant if there happens to be a memory-mapped hardware device at the target address.


So which register actually gets written to memory? This is fixed in some later chips like the 65SC02 so for compatibility always ensure the indirect vector is not at the end of the page.

The addressing modes are the 652 as the 10 case, except that accumulator mode is missing. But if we rearrange it so that columns with the same bits are close together, correlations become easier to see:.

The 6502/65C02/65C816 Instruction Set Decoded

Notably this is not related in any way to the state of the carry bit of the accumulator. July 27, Modified: Instead of entries telling how to process each separate opcode, it’s encoded as combinational logic post-processing the output of a “sparse” Opodes that acts in some ways like a programmable logic array PLA. Signed values are two’s complement, sign in bit 7 most significant bit.

Unofficial opcodessometimes called illegal opcodes or undocumented opcodesare CPU instructions that were officially left unused by the original design.