User's Manual

PMAC User Manual
274 Writing a Host Communications Program
To reassemble a long fixed-point word in the host, take the less significant 32-bit word, and mask out the
sign extension (top eight bits). In C, this operation could be done with a bit-by- bit AND: (LSW &
16777215). Treat this result as an unsigned integer. Next, take the more significant word and multiply it
by 16,777,216. Finally, add the two intermediate results together.
To reassemble a long floating-point value in the host, first split the 64-bit value into its pieces. Bits 0 to
11 of the 32-bit word at the lower address form the exponent. Bits 12-23 of this word form the low 12
bits of the 36-bit mantissa. Bits 0-23 of the 32-bit word form the high 24 bits of the mantissa; bits 24-31
are sign extension. The following code shows one way of creating these pieces (not the most efficient):
exp = first_word & 0x00000FFFL; // Select low 12 bits for exponent
low_mantissa = (first_word >> 12) & 0x00FFFL;
// Select next 12 bits for mantissa
// shift right and mask high_mantissa = second_word;
The floating point value can then be reconstructed with:
mantissa = high_mantissa * 4096.0 + low_mantissa;
value = mantissa * pow (2.0, exp - 2047 - 35);
2047 is subtracted from the exponent to convert it from an unsigned to a signed value; 35 is subtracted to
put the mantissa in the range of 0.5 to 1.0.
The following procedure in C performs the conversion efficiently and robustly:
double DPRFLoat (long first_word, long second word) {
// return mantissa/2^35 * 2^(exp-2047)
double mantissa;
long int exp;
m = (double)(second_word) * 4096.0 +
(double)((first_word>>12) & 0x00FFFL);
if (m == 0.0) return (0.0);
exp = (first_word & 0x00000FFFL) - 2082; // 2082=2047+35
return (mantissa * pow(2.0 * (double) exp));
}
To reassemble a long floating-point word in the host, treat the less significant word the same as for the
fixed-point case above. Take the bottom 12 bits of the more significant word (MSW & 4095), multiply
by 16,777,216 and add to the masked less significant word. This forms the mantissa of the floating-point
value. Now take the next 12 bits (MSW & 16773120) of the more significant word. This is the exponent
to the power of two, which can be combined with the mantissa to form the complete value.
Byte 2 Byte 1 Byte 0
Byte 2
Byte 1
Byte 0
Byte 2 Byte 1 Byte 0
Byte 5 Byte 4
Byte 2
Byte 1
Byte 0
Byte 4
Byte 5
1
11
1
1
1
1
1
S S S S S S S S
S
S
1
S
S
2
1
S
S
2
07 8 151623
07
8 15 1623
BIT #
BIT #
DPRAM
BYTE
NO.
(RELATIVE)
0
1
2
3
24 BITS
PMAC WORD:
48 BITS
PMAC Y: WORD
PMAC X: WORD
DPRAM
BYTE
NO.
(RELATIVE)
0
1
2
3
4
5
6
7
(SIGN EXTENSION)
(SIGN EXTENSION)
(SIGN EXTENSION)
S = Sign bit
1
S
= First word sign bit
= Second word sign bit
= Exponent for floating point
S
2
Byte 3
Byte 3
S
2
S
2
S
2
S
2
S
2
S
2
S
2
S
2
S
SS S
SS S
S
Figure 57 Dual-ported RAM Data Gathering Format