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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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. A small number of games use them see below. Though the instruction set has a number of quirks and irregularities, large portions of it can be broken up into regular patterns. 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.
The aaa bits determine the opcode as follows:. Generally, instructions of a kind are typically found in rows as a combination of a and cand address modes are in columns b. Actually, it’s not quite correct to say that these instructions 6052 do anything with their operands. This instructions is used to test if one or more bits are set in a target memory location.
An inclusive OR is performed, bit opcoddes bit, on the accumulator contents using the contents of a byte of memory.
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.
The use of unofficial opcodes is rare in NES games. Shown below are the instructions of the65C02, and 65C processors. These instructions are missing on 65C02s made o;codes other manufacturers. Retrieved from ” https: There are also some useful documents at Western Design Center. CPU unofficial opcodes From Nesdev wiki. I’ve included them 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.
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. Most of these are put to work supplying the new long addressing modes of the 65C This instruction subtracts the contents of a memory location to the accumulator together with the not of the carry bit.
The columns are colored by bits 1 and 0: May 29, Added a new note about 65C02 “undocumented” opcodes. This causes instructions to have strange mixing properties. The flag indicated by xx is compared with yand the branch is taken if they are equal.
If overflow occurs the carry bit is set, this enables multiple byte addition to be performed.
Note that the discussion below assumes a knowledge of programming. The bit manipulation instructions found only on the Rockwell and WDC versions opcdoes the 65C02 are not included in the table, nor are the “undocumented” instructions of the original The instruction table is laid out according to a pattern a-b-c, where a and b are an octal number each, followed by a group of two binary digits c, as in the bit-vector “aaabbbcc”.
Bit 7 is set to zero.
This table lists all opcodes, 32 columns per row. But apparently this isn’t always reliable–there are reports of some of these instructions occasionally locking up the processor. The aaa and cc bits determine the opcode, and the bbb bits determine the addressing mode.
An original has does not correctly fetch the target address if the indirect vector falls on a page boundary e. Mind that the two notations are interchangeable for any instructions involving the accumulator. Adds one to the value held at a specified memory location setting the zero and negative flags as appropriate.
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. 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. Subtracts one from the value held at a specified memory location setting the zero and negative flags as appropriate. Some instructions landed in logical places, but others had to be assigned wherever there was room, whether it made sense or not.
In some cases the 01 and 10 instructions are incompatible. 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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Some even differ based on analog effects. This instruction compares the contents of the X register with another memory held value and sets the zero and carry flags as appropriate.
Bits 7 and 6 of olcodes value from memory are copied into the N and V flags. Move each of the bits in either A or M one place to the right. If the zero flag is clear then add the relative displacement to the program counter to cause a branch to a new location.
An accurate NES emulator must implement all instructions, not just the official ones. 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.
So which register ipcodes gets written to memory?