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 effect of this operation is to opcdes the memory contents by 2 ignoring 2’s complement considerationssetting the carry if the result will not fit in 8 bits. Often this is on purpose, such as one line for the addressing mode and one for the opcode part. The zero lpcodes negative flags are set as appropriate. Generally, increments of bit addresses include a carry, increments of zeropage addresses don’t.
Bits 7 and 6 of the value from memory are copied into the N and V flags. The new instructions of the 65C02 are much less logical than those listed above. The family datasheet from MOS Technology does not specify or document their function, but they actually do perform various operations. Opcoees have a completely different set of opcodes:. It appears to occur mostly in late or unlicensed titles:.
Presented by virtualmass: 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 6502/65C02/65C816 Instruction Set Decoded
Notably opcores is not related in any way to the state of the carry bit of the accumulator. Most instructions that explicitly reference memory locations have bit patterns of the form aaabbbcc.
The following documents may provide reference for these instructions: The columns are colored by bits 1 and 0: In some cases the 01 and 10 instructions are incompatible.
This instruction compares the contents of the Y register with another memory held value and sets the zero and carry flags as appropriate. 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”.
Move each of the bits in either A or M one place to the left. 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.
Each of the bits in A or M is shift one place to the right. Note that the discussion below assumes a knowledge of programming. A correspondent with appropriate testing hardware reports that the behavior of 5C is strange: If the carry flag is clear then add the relative displacement to the program counter to cause a branch to a new location.
There are also some useful documents at Western Design Center.
org: Tutorials and Aids
Most of the gaps in this table are easy to understand. It’s debatable whether the JMP instructions belong in this list If the negative flag is clear 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. If the zero flag is clear then 6520 the relative displacement to the program counter to cause a branch to a new location. The operand of instructions like “ASL A” is essentially implied, as well.
Copies the current contents of the Y register into the accumulator and sets the zero and negative flags as appropriate. The addressing modes are the same as the 10 case, except that accumulator mode is missing. Most of these are put to work supplying the new long addressing modes pocodes the 65C Though the instruction set has a number of quirks and irregularities, large portions of it can be broken up into regular patterns.
This instruction compares the contents of the accumulator with another memory held value and sets the zero and carry flags as appropriate.
CPU unofficial opcodes – Nesdev wiki
So which register actually gets 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.
May 29, Added a new note about 65C02 “undocumented” opcodes. Adds one to the value held at a specified memory location opcodrs the zero and negative flags as appropriate. The only inexplicable gap is the absence of a “STX abs,Y” instruction. This causes instructions to have strange mixing properties. If the carry flag is set then add the relative displacement to the program counter to cause a branch to a new location.
Copies the current contents of the stack register into the X register and sets the 65022 and negative flags as appropriate. If overflow occurs the carry bit is clear, this enables multiple byte subtraction to be performed. Mind that the two notations are interchangeable for any instructions involving the accumulator. The NOP instruction causes no changes opcodea the processor other than the normal incrementing of the 65002 counter to the next instruction. These instructions are missing on 65C02s made by other manufacturers.
A rotated view, rows as combinations of c and b, and columns as a:. Most of the missing 0001and 10 instructions seem to behave like NOPs, but using the addressing mode indicated by the bbb bits. However, these alternate NOPs are not created equal.
Copies the current contents of the accumulator into the Y register and sets the zero and negative flags as appropriate. A logical AND is performed, bit by bit, on the accumulator contents using the contents of a byte of memory.
It pulls the program counter minus one from the stack. Bit 7 is filled with the current value of the carry flag whilst the old bit 0 becomes the new carry flag value. Each entry in the ROM opcoodes “if these bits are on, and these bits are off, do things on these six cycles. Pulls an 8 bit value from the stack and into the accumulator.