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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If the carry flag is clear then add the relative displacement to the program counter to cause a branch to a new location. This instruction compares the contents of the Y register with another memory held value and sets the zero and carry flags as appropriate. 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.
Copies the current contents of the stack register into the X register and sets the zero and negative flags as appropriate. The aaa bits determine the opcode as follows:. Actually, it’s not quite correct to say that these instructions don’t do anything with their operands.
Opcodes – Plus/4 Encyclopedia – Plus/4 World
But if we rearrange it so that columns with the same bits are close together, correlations become easier to see:. Some instructions landed in logical places, but others had to be assigned wherever there was room, whether it made sense or not. 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.
Bits 7 and 6 of the value from memory are copied into the N and V flags. The use of unofficial opcodes is rare in NES games. Subtracts one from the value held at a specified memory location setting the zero and negative flags as appropriate.
If the carry flag is set then add the relative displacement to the program counter to cause a branch to a new location.
This instruction subtracts the contents of a memory location to the accumulator together with the not of the carry bit. However, these alternate NOPs are not created equal.
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. Most NMOS cores interpret them the same way, although there are slight differences with the less stable instructions. Copies the current contents of the Y register into the accumulator and sets the zero and negative flags as appropriate.
The Instruction Set Decoded
Pulls an 8 bit value from the stack and into the processor flags. An exclusive OR is performed, bit by bit, on the accumulator contents using the contents of a byte of memory. Instead of entries telling how to process each separate opcode, it’s encoded as combinational logic post-processing the output of a “sparse” Opcodew that acts in some ways like a programmable logic array PLA.
Move 65022 of the bits in either A or M one place to the left. Presented by virtualmass: CPU unofficial opcodes From Nesdev wiki.
6502 Opcodes
It pulls the processor flags from the stack followed by the program counter. The zero and negative flags are set as appropriate.
Some have one- or two-byte operands which they don’t do anything withand they take different amounts of time to execute. Though the instruction set has a number of quirks and irregularities, large portions of it can be broken up into regular patterns.
Each of the bits in A or M is shift one place to the right. If the overflow flag is clear then add the relative displacement to the program counter to cause a branch to a new location. And since 65002 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.
In both cases you should include an explicit CLD to ensure that the flag is cleared before performing addition or subtraction. This causes instructions to have strange mixing properties. So which register actually gets written to memory?
6502 Instruction Set
The bit manipulation instructions found only on the Rockwell and WDC versions of the 65C02 are not included in the table, nor are the “undocumented” instructions of the original A small number of games use them see below.
The only inexplicable gap is the absence of a “STX abs,Y” instruction. The columns are colored by bits 1 and 0: Load, store and transfer instructions as well as comparisons are typically found in the lower half of the table, while most of the arithmetical and logical operations as well as stack and jump instructions are found in the upper half. An accurate NES emulator must implement all instructions, not just the official ones.