MIPS MIPS32 74K Series, Руководство по программированию

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6.2 Basic instruction set
Programming the MIPS32® 74K™ Core Family, Revision 02.14 78
Figure 6.1 How floating point numbers are stored in a register
Just to remind you:
• sign: FP numbers are positive numbers with a separate sign bit; “1” denotes a negative number.
• mantissa: represents a binary number. But this is a floating point number, so the units depend on:
• exp: the exponent.
When 32-bit data is held in a 64-bit register, the high 32 bits are don’t care.
The MIPS Architecture’s 32-bit and 64-bit floating point formats are compatible with the definitions of “single preci-
sion” and “double precision” in
[IEEE754] .
FP registers can also hold simple 2s-complement signed integers too, just like the same number held in the integer
registers. That happens whenever you load integer data, or convert to an integer data type.
Floating point data in memory is endianness-dependent, in just the same way as integer data is; the higher bit-num-
bered bytes shown in
Figure 6.1 will be at the lowest memory location when the core is configured big-endian, and
the highest memory location when the core is little-endian.
6.2 Basic instruction set
Whenever it makes sense to do so, FP instructions exist in a version for each data type. In assembler that’s denoted by
a suffix of:
There’s a good readable summary of the floating point instruction set in [SEEMIPSRUN]: , and you can find the fine
technical details in [MIPS64V2] .
As a one-minute guide: the FPU provides basic arithmetic (add, multiply, subtract, divide and square root). It’s all
register-to-register (like the integer unit). It’s written “destination first” like integer instructions; sometimes that’s
unexpected in that
cvt.d.s
is a “convert from single to double”. It has a set of multiply/add instructions which
work on four registers:
madd a,b,c,d
does
a = c*d + b
.s single-precision
.d double-precision
.w 32-bit integer (“word”)
.l 64-bit integer
float
double
16 23 8 15 0 7
mantissa sign exp
24 31
56 63 32 39 40 47 48 55 16 23 24 31 8 15 0 7
mantissa sign exp