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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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The following table lists the instruction set, rows sorted by c, then a.

If the overflow flag is clear then add the relative displacement to the program counter to cause a branch to a new location. Notably this is not related in any way to the state of the carry bit of the accumulator. May 3, Modified: Increments without carry do opcpdes affect the hi-byte of an address and no page transitions do occur.

Thus the 00 red block is mostly control instructions, 01 green is ALU operations, and 10 blue is read-modify-write RMW operations and data movement instructions involving X. A small number of games use them opcdes below.

Bit 7 is filled with the current value of the carry flag whilst the old bit 0 becomes the new carry flag value. This instructions is used to test if one or more bits are set in a target memory location. Mind that the two notations are interchangeable for opocdes instructions involving the accumulator.


The Instruction Set Decoded

Address modes are either a property of b even columns or combinations of b and c odd columns with aspecific row-index modulus 3; i. The RTI instruction is used at the end of an interrupt processing routine.

The only inexplicable gap is the absence of a “STX abs,Y” instruction. Bit 0 is set to 0 and bit 7 is placed in the carry flag.

The 6502/65C02/65C816 Instruction Set Decoded

Note that the discussion below assumes a knowledge of programming. If the zero flag is clear then add the relative displacement to the program counter to cause a branch to a new location. Presented by virtualmass: The instruction table is laid out according to a pattern opcoeds, where a and b are an octal number each, followed by a group of two binary digits c, as in the bit-vector “aaabbbcc”.

If the carry flag is clear then add the relative displacement to the program counter to cause a branch to a new location. But the above assigments exhaust the logical possibilities for opcodes that explicity reference memory locations, so TRB and STZ had to be put wherever room could be found:.

Subtracts one from the opcoddes held at a specified memory location setting the zero and negative flags as appropriate.

The question often arises, “What do all those other leftover bytes do if you try to execute them as instructions? Bits 7 and 6 of the value from memory are copied into the N and V flags. The columns are colored by bits 1 and 0: An accurate NES emulator must implement all instructions, not just the official ones. Some have one- or two-byte operands which they don’t do anything withand they take different amounts of time to execute.


Most instructions that explicitly reference memory locations have bit patterns of the form aaabbbcc. Each entry in the ROM means “if these bits are on, and these bits are off, do things on these six cycles.

So which register actually gets written to memory? It appears to occur mostly in late or unlicensed titles:. In both cases you should include an explicit CLD to ensure that the flag is cleared before performing addition or subtraction.

Page transitions may occur and add an extra cycle to the exucution. Unofficial opcodessometimes called illegal opcodes or undocumented opcodesare CPU instructions that were officially left unused by the original design. There aren’t really any undocumented instructions on the 65Cany instructions not listed above are documented as performing no operation. This causes instructions to have strange mixing properties. However, there are some facts that seem to be common across all s.