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 6502/65C02/65C816 Instruction Set Decoded
The effect of this operation is to multiply the memory contents by 2 ignoring 2’s complement considerationssetting the carry if the result will not fit in 8 bits. But apparently this isn’t always reliable–there are reports of some of these instructions occasionally locking up the processor.
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 negative flag is clear then add the relative displacement to the program counter to cause a branch to a new location.
Bit 7 is filled with the current value of the carry flag whilst the old bit 0 becomes the new carry flag value.
In some cases the 01 and 10 instructions are incompatible. Each of the bits in A or M is shift one place to the right. The microcode of the is compressed into a entry decode ROM. 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. Note that the discussion below assumes a knowledge of programming. If overflow occurs the carry bit is set, this enables multiple opcodfs addition to be performed.
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 carry flag is set then add the relative displacement to the program counter to cause a branch to a new location. Bit 7 is set to zero.
I’ve included opcoces because they do seem to fit, provided one considers the indirect JMP a separate opcode rather than a different addressing mode of the absolute JMP. The columns are colored by bits 1 and 0: This instruction compares the contents of the Y register with another memory held value and sets the zero and carry flags as appropriate.
CPU unofficial opcodes – Nesdev wiki
The RTI instruction is used at the end of an interrupt processing routine. Presented by virtualmass: This instruction subtracts the contents of a memory location to the accumulator together with the not of the carry bit.
Most NMOS cores interpret them the same way, although there are slight differences with the less stable instructions. Copies the current contents of the X register into the accumulator and sets the zero and negative flags as appropriate. If the zero flag is set then add the relative displacement to the program counter to cause a branch to a new location.
This instruction compares the contents of the X register with another memory held value and sets the zero and carry flags as appropriate.
Bit 0 is set to 0 and bit 7 is placed in the carry flag. Usually some mixture of the two, in a manner that varies depending on who made thewhen it was made, the phase of the moon, and other unpredictable variables.
Bit 0 is filled with the current value of the carry flag whilst the old bit 7 becomes the new carry flag value. The new instructions of the 65C02 are much less logical than those listed above.
There are also some useful documents at Western Design Center. 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. Generally, increments of bit addresses include a carry, increments of zeropage addresses don’t.
Perhaps the pattern is easier to see by shuffling the ‘s opcode matrix. Copies the current contents of the accumulator into the Y register and sets the zero and negative flags as appropriate.
On 65C02s made by Rockwell and by WDC, some of these instructions are used for additional bit setting, clearing, and testing instructions. Increments without carry do not affect the hi-byte of an address and no page transitions do occur. An accurate NES emulator must implement all instructions, not just the official ones.
Subtracts one from the value held at a specified memory location setting the zero and negative flags as appropriate. Actually, opodes not quite correct to say that these instructions don’t do anything with their operands. Move each of the bits in either A or M one place to the left.
6502 Instruction Set
Signed values are two’s complement, sign in bit 7 most significant bit. 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. But many of the unofficial opcodes simultaneously trigger parts of the ROM that were intended for completely unrelated instructions.
Most of these are put to work supplying the new long addressing modes of the 65C The addressing modes are the same as the 10 case, except that accumulator mode is missing. The state of the decimal flag is uncertain when the CPU is powered up and it is not reset when an interrupt is generated. The aaa and cc bits determine the opcode, and the bbb bits determine the addressing mode. An exclusive OR is performed, bit by bit, on the accumulator contents using oppcodes contents of a byte of memory.
The aaa bits determine the opcode as follows:. If overflow occurs the carry bit is clear, this enables multiple byte subtraction to be performed. CPU unofficial opcodes From Nesdev wiki. Those looking for a precise listing of “undocumented” instruction behaviors will have to look elsewhere, and should beware that the behaviors described on other web pages may be specific to s made by a particular often unspecified manufacturer.
Opcoves 29, Added a new note about 65C02 “undocumented” opcodes.