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→‎Known Bugs: Added a clearer example of the MUL/MUL CPU bug
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= 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 [https://en.wikipedia.org/wiki/Stepping_level steppings], and affects someearly but not allmodel Nintendo 64 consoles. Theregenerally isNUS-01 a(Japan ROMOnly), whichNUS-02 tests(Japan ifOnly), yourNUS-03 hardware(First isUS affectedRevision). by this bug (where?)
 
GCC accepts the <code>-mfix4300</code> flag, which tells GCC to generate code with a workaround for this bug—two <code>nop</code> instructions are inserted after every <code>mul.s (fp),</code> or <code>mul.d (fp) or mult (integer)</code>. For example, consider this function:
 
For example, consider this function:
float mul(float x, float y) { return x * y; }▼
 
▲ <code>float mul(float x, float y) { return x * y; }</code>
If the fix is disabled, GCC will generate the following assembly (for the n32 ABI):
Without the fix it may generate this code:
 
<code>
# Does not contain workaround, may cause problems.
jal mul:
jr $31nop
mul.s $f0f1,$f12f13,$f14f15
mul:▼
 
jr $31▼
With the setting enabled, GCC will append <code>nop</code> instructions. Note that in this case, the second one goes into the branch delay slot.
mul.s $f0,$f12,$f14▼
 
</code>
# Contains workaround for CPU bug.
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.
▲ mul:
▲ mul.s $f0,$f12,$f14
nop▼
▲ jr $31
nop▼
 
With the fix it may generate this code:
<code>
mul.s $f1,$f13,$f15
▲ nop
jal mul
▲ nop
nop
mul:
mul.s $f0,$f12,$f14
jr $31
nop
</code>
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.
[[Category:Motherboard components]]
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