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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Finally, a more complex view, the instruction set listed by opcodea as combinations of a and c, and b in columns: If the overflow flag is set then add the relative displacement to the program counter to cause a branch to a new location. This causes instructions to have strange mixing properties.
However, these alternate NOPs are not created equal. Unofficial opcodessometimes called illegal opcodes or undocumented opcodesare CPU instructions that were officially left unused by the original design. Bit 0 is set to 0 and bit 7 is placed in the carry flag.
Opcodes – NES Hacker Wiki
The mask pattern in A is ANDed with the value in memory to set or clear the zero flag, but the result is not kept. It 65022 to occur mostly in late or opcoees titles:. If the carry flag is set then add the relative displacement to the program counter to cause a branch to a new location. So which register actually gets written to memory? Subtracts one from the value held at a specified memory location setting the zero and negative flags as appropriate.
Copies the current contents of the Y register into the accumulator and sets the zero and negative flags as appropriate.
An increment with carry may affect the hi-byte and may thus result in a crossing of page boundaries, adding an extra cycle to 6052 execution.
CPU unofficial opcodes – Nesdev wiki
Opcoves microcode of the is compressed into a 65502 decode ROM. Copies the current contents of the stack register into the X register and sets the zero and negative flags as appropriate. Actually, it’s not quite correct to say that these instructions don’t do anything with their operands.
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:. An inclusive OR is performed, bit by bit, on the accumulator contents using the contents of a byte of memory.
The 6502/65C02/65C816 Instruction Set Decoded
652 The JSR instruction pushes the address minus one of the return point on to the stack and then sets the program counter to the target memory address. The bit that was in bit 0 is shifted into the carry flag. Opcdes NOP instruction causes no changes to the processor other than the normal incrementing of the program counter to the next instruction.
Each of the bits in A or M is shift one place to the right. Some of these instructions are useful; some are not predictable; some do nothing but burn cycles; some halt the CPU until reset. An understanding of these patterns can be beneficial to authors of assemblers or disassemblers for code–for example, the Apple II ROM uses the information described below to greatly reduce the size of the instruction tables used by the built-in machine language disassembler.
CPU unofficial opcodes From Nesdev wiki. The remaining instructions are probably best considered simply by listing them.
Signed values are two’s complement, sign in bit 7 most significant bit. Notably this is not related in any way to the state of the carry bit of the accumulator. Copies the current contents of the accumulator into the X register and sets the zero and negative flags as appropriate.
An exclusive OR is performed, bit by bit, on the accumulator contents using the contents of a byte of memory. 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 opxodes the memory location to set or clear. Shown below are the instructions of the65C02, and 65C processors.
The only inexplicable gap is the absence of a “STX abs,Y” instruction. If the zero flag is set then add the relative displacement to the program counter to cause a branch to a new location.
If the negative flag is clear then add the relative displacement to the program counter to cause a branch to a new location. Generally, instructions of a kind are typically found in rows as a combination of a and cand address modes are in columns b. Similarly, the test-and-branch instructions are 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.
Here are the interrupt and subroutine instructions:. The conditional branch instructions all have the form xxy Most of the gaps in this table are easy to understand.
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. Adds one to the value held at a specified memory location setting the zero and negative flags as appropriate. 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.
JSR is the only absolute-addressing instruction that doesn’t fit the aaabbbcc pattern.
On 65C02s made by Rockwell and by WDC, some of these instructions are used for additional bit setting, clearing, and testing instructions. Most of these are put to work supplying the new long addressing modes of the opckdes The question often arises, “What do all those other leftover bytes do if you try to execute them as instructions?