What is the general format of TurboGrafX processor instructions?

For example, and this is made up: one word for opcode + two words for flags + one word for every parameter listed under that instruction.

Please be specific.

  • 2
    Have you looked at the 6280 instruction set? The general format is 1 byte for the opcode, 0-6 bytes for the operand(s). Oct 17, 2018 at 2:40
  • Does the TurboGrafX modified version have any noticeable differences?
    – Badasahog
    Oct 17, 2018 at 20:55
  • also, where do the flags go?
    – Badasahog
    Oct 18, 2018 at 15:21
  • 1
    The flags are not part of the instruction, they are found in the P register, and the current value of this when the instruction is executed may alter the behavior of the instruction. Various instructions may set or read some of the flags. For example, SED and CLD clear the "decimal" flag, which decides whether a later ADC instruction will work in binary or BCD mode. Oct 20, 2018 at 17:40
  • 1
    (sorry, should have said SED and CLD set or clear the D ("decimal") flag respectively) Oct 21, 2018 at 17:08

1 Answer 1


The HuC6280 used an instruction set and architecture which was largely identical to the 65C02 -- each instruction is represented by a one-byte opcode, followed by up to three bytes of immediate data. The amount of immediate data is implied by the opcode.

(The flags are a CPU register, not part of an instruction.)

The HuC6280 appears to support all instructions which existed on the 65C02, plus some additional instructions. Some of these instructions are specific to the PC-Engine hardware, while others are more general-purpose:

  • BSR: A form of JSR which uses a relative address.

  • CLA, CLX, CLY: One-byte instructions to set specific registers to zero.

  • CSH, CSL: Instructions to change the speed of the processor.

  • SAX, SAY, SXY: Swap values between the A, X, and Y registers.

  • SET: Acts as a prefix, causing the next instruction to use (X) -- that is, a zero-page memory value -- instead of A.

  • ST0, ST1, ST2: Store immediate values to VDC control registers.

  • TAM, TMA: Store and retrieve values to/from memory mapper registers.

  • TIA, TDD, TIN, TII: Control memory-move operations.

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