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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Xxxx instructions are also problematic–some of these seem to mix not only the adjacent 01 and 10 instructions, but also the immediate mode of the corresponding 10 instruction. This operation shifts all the bits of the accumulator or memory contents one bit left. The JSR instruction pushes the address minus one of the return point on to the stack and then sets the program counter ppcodes the target memory address.

Bit 0 is set to 0 and bit 7 is placed in the carry flag.

6502 Opcodes

The columns are opcoxes by bits 1 and 0: Move each of the bits in either A or M one place to the right. Most of the gaps in this table are easy to understand.

Bit 0 is filled with the current value of the carry flag whilst the old bit 7 becomes the new carry flag value. The following table lists the instruction set, rows sorted by c, then a. Personal tools Create account Log in. If the overflow flag is clear then add the relative displacement to the program counter to cause a branch to a new location. Copies the current contents of the stack register into the X register and sets the zero and negative flags as appropriate.

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Copies the current contents of the accumulator into the Y register and sets the zero and negative flags as appropriate. An accurate NES emulator must implement all instructions, not just the official ones.

6502 Instruction Set

And since this page is part of a set of Apple II-related pages, I should point out that Apple never shipped any computers that used Rockwell or WDC 65C02s, so none of the instructions in this section are available on an unmodified Apple II.

If the zero flag is clear then add the relative displacement to the program counter to cause a branch to a new location. However, these alternate NOPs are not created equal. This instruction compares the contents of the accumulator with another memory held value and sets the zero and carry flags as appropriate.

It pulls the processor flags from the stack followed by the program counter.

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The behavior of the 11 instructions is especially problematic in those cases where the adjacent 01 or 10 instruction is also undocumented. Signed values are two’s complement, sign in bit 7 most significant bit. The NOP instruction causes no changes to the processor other than the normal incrementing of the program counter to the next instruction.

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Each entry in the ROM means “if these bits are on, and these bits are off, do things on these six cycles. Most instructions that explicitly reference memory locations have bit patterns of opcodee form aaabbbcc. The remaining instructions are probably best considered simply by listing them.

An inclusive OR is performed, bit by bit, on the accumulator contents using the contents of a byte of memory. It pulls the program counter minus opocdes from the stack.

The conditional branch instructions all have the form xxy If the zero flag is set then add the relative displacement to the program counter to cause a branch to a new location. Most NMOS cores interpret them the same way, although there are slight differences with the less stable instructions.

Finally, a more complex view, the instruction set listed by rows as combinations of a and c, and b in columns: The question often arises, “What do all those other leftover bytes do if you try to execute them as instructions? Mind that the two notations are interchangeable for any instructions involving the accumulator. Some instructions landed in logical places, but others had to be assigned wherever there was room, whether it made sense or not. The RTI instruction opcodess used at the end of an interrupt processing routine.