Both the exponent and mantissa have separate sign bits. All digits are packed BCD, and
the entire string fits in 96 bits (three long words). As is the case with all data formats, when
packed BCD strings are supplied to the FPCP, the strings are automatically converted to
extended precision real values. This conversion allows packed BCD numbers to be used
as inputs to any operation. For example:
FADD.P #-6.023E + 24,FP5
BCD numbers can be supplied by the FPCP in a format readily used for printing by a
program generated by a high-level language compiler. For example:
FMOVE.P FP3,BUFFER{#- 5}
This instruction converts the contents of floating-point data register 3 (FP3) into a packed
BCD string with five significant digits to the right of the decimal point (FORTRAN F format).
1.3.4 Data Format Summary
All data formats described in the preceding paragraphs are supported orthogonally by all
arithmetic and transcendental operations and by all appropriate MPU addressing modes.
For example, all of the following are valid instructions:
FADD.B #0,FP0
FADD.W D2,FP3
FADD.L BIGINT, FP7
FADD.S #3.14159,FP5
FADD.D (SP)+ ,FP6
FADD.X [(TEMP PTR,A7)],FP3
FADD.P #1.23E25,FP0
Most on-chip calculations are performed in the extended precision format, and the eight
floating-point data registers always contain extended precision values. All operands are
converted to extended precision by the FPCP before a specific operation is performed, and
all results are in extended precision. This ensures maximum accuracy without sacrificing
performance.
Refer to Figure 1-10 for a summary of the memory formats for the seven data formats
supported by the FPCP.
1.4 INSTRUCTION SET
The FPCP instruction set is organized into six major classes:
1. Moves between the FPCP and memory or the MPU (to or from)
2. Move multiple registers (to or from)
3. Monadic operations
4. Dyadic operations
5. Branch, set, or trap conditionally
6. Miscellaneous
1.4.1 Moves
On all moves from memory (or from an MPU data register) to the FPCP, data is converted
from the source
data
format to the internal extended precision format. On all moves from
FREESCALE
1-12
MC68881/MC68882 USER'S MANUAL
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