VR4300

The VR4300 is the CPU of the Nintendo 64, and is a NEC VR4300 with slight modifications. Running at 93.75 MHz, the VR4300 handles game logic, reading responding to controller input from the PIF, and preparing display lists and audio command lists for the RCP to draw and synthesize audio.

VR4300 Users Manual

= Naming History = In 1993, MIPS Technologies, Inc. (MTI), the same company that designed the MIPS III architecture, developed the R4200 microprocessor. Later in 1995, they made the R4300i, a derivative of the R4200. When MTI licensed it to NEC and Toshiba, those companies renamed the chips the VR4300 and TX4300 respectively. The microprocessor used in the N64's CPU, is a derivative of NEC's VR4300.

Over the years, these model names have been incorrectly identified. There is no such thing as a VR4300i, nor is there a NEC R4300.

= Known Bugs =

Some VR4300 CPUs contain the “VR4300 multiplication bug”. This causes incorrect results to be generated, under certain circumstances, after computing a floating-point multiplication. The bug was fixed in later processor steppings, and affects early model Nintendo 64 consoles generally NUS-01 (Japan Only), NUS-02 (Japan Only), NUS-03 (First US Revision).

GCC accepts the  flag, which tells GCC to generate code with a workaround for this bug—two   instructions are inserted after every ,  , or.

For example, consider this function: Without the fix it may generate this code: jal mul nop mul.s  $f1,$f13,$f15 mul: jr     $31 mul.s  $f0,$f12,$f14 The mul.s after the nop (red) may produce unexpected results, if the operands in the mul.s after the jr (yellow) include NaN, Zero or Infinity.

With the fix it may generate this code: mul.s  $f1,$f13,$f15 nop jal mul nop nop mul: mul.s  $f0,$f12,$f14 jr     $31 nop Depending on the other instructions that can be reordered the nops could be other instructions that perform work, so this is a worst case scenario. There may be a ROM which tests if your hardware is affected by this bug, but determining based on the Motherboard revision is easier.