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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Opfodes 65C02s made by Rockwell and by WDC, some of these instructions are used for additional bit setting, clearing, and testing instructions. 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.
The columns are colored by bits 1 and 0: The microcode of the is compressed into a entry decode ROM. Most of the missing 0001and 10 instructions seem to behave like NOPs, but using the addressing mode indicated by the bbb bits. The use of unofficial opcodes is rare in NES games.
If the negative flag is clear then add the relative displacement to the program counter to cause a branch to a new location. Some instructions landed in logical places, but others had to be assigned wherever there was room, whether it made sense or not. Actually, it’s not quite correct to say that these instructions don’t do anything with their operands. This instruction compares the contents of the X register with another memory held value and sets the zero and carry flags as appropriate.
Personal tools Create account Log in. Finally, a more complex view, the instruction set listed by rows as combinations of a and c, and b in columns: Some have one- or two-byte operands which they don’t do anything withand they take different amounts of time to execute.
6502 Instruction Set
The RTI instruction is used at the end of an interrupt processing routine. Olcodes conditional branch instructions all have the form xxy A small number of games use them see below. An inclusive OR is performed, bit by bit, on the accumulator contents using the contents of a byte of memory. 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.
If the carry flag is clear then add the relative displacement to the program counter to cause a branch to a new location. 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.
This causes instructions to have strange mixing properties. Bit 0 is set to 0 and bit 7 is placed pocodes the carry flag. 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.
Most of the gaps in this table are easy to understand. But if we rearrange it so that columns with the same bits are close together, correlations 66502 easier to see:.
Opcodes – NES Hacker Wiki
However, there are some facts that seem to be common across all s. Unofficial opcodessometimes called illegal opcodes or undocumented opcodesare CPU instructions that were officially left unused by the original design. Though the instruction set has a number of quirks and irregularities, large portions of it can be broken up into regular patterns. An original has does not correctly fetch the target address if the indirect vector falls on a page boundary e.
The state of the decimal flag is uncertain when the CPU is powered up and it is not reset when an interrupt is generated.
Copies the current contents of the accumulator into the Y register and sets the zero and negative flags as appropriate. Some even differ based on analog effects. JSR is the only absolute-addressing instruction that doesn’t fit the aaabbbcc pattern.
Instructions of the form xxxx usually lock up the processor, so that a reset is required to recover. So which register actually opcodea written to memory? The behavior of the 11 instructions is especially problematic in those cases where the adjacent 01 or 10 instruction is also undocumented. The aaa bits determine the opcode as follows:.