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SC140 DSP Core Reference Manual
Instruction Set Overview
6.2.2 Serial Grouping
In the serial grouping method, the two most significant bits of each instruction in the execution set provide
the core with the necessary information to perform instruction grouping.
Each SC140 instruction belongs to one of the following four types:
Type 1
— Basic DALU and move instructions, which are frequently used single-word instructions.
Type 2
— Additional DALU, move, and AGU arithmetic instructions that are also single-word
instructions, but not used as frequently as Type 1 instructions.
Type 3
— Two-word and three-word DALU, move, and AGU arithmetic instructions.
Type 4
— All other instructions, which may be one or two words long.
The serial grouping options for an execution set are:
One to six Type 1 instructions.
One Type 2 instruction grouped with up to five Type 1 instructions.
One Type 3 instruction grouped with up to five Type 1 instructions.
One Type 4 instruction.
The two MS bit combinations that characterize serially grouped Type 1 instructions are:
00 for instructions grouped with the next instruction.
01 to indicate the last Type 1 instruction in the set.
In serial grouping (by definition), Type 2, 3, and 4 instructions terminate an execution set. Type 4
instructions cannot be grouped with any other instruction.
6.2.3 Prefix Grouping
A one-word or two-word prefix that is placed at the beginning of an execution set can convey additional
information about that execution set. The detailed encoding for prefix words is found in the front of
Appendix A. Prefix words carry information on the following topics:
The number of instructions that are grouped together in the execution set.
Conditional execution of the whole set or a subgroup of the set, originating in IFT/IFF/IFA.
Looping information for supporting hardware loops (encoding the LPMARKA and LPMARKB
mnemonics).
Encoding extension for high register banks (D8-D15, R8-R15).
Information concerning the first three items above is encoded in the one-word prefix. The two-word prefix
includes all information in the one-word prefix as well as the encoding of the high bank registers.
The grouping information held in the prefix includes the number of words to be grouped (including the
number of prefix words) minus one. Valid values are from 0 to 7. A value of 0 corresponds to a NOP
instruction that is not dispatched.
The basic SC140 16-bit instruction encoding allocates only three bits for specifying a data or pointer
register. On their own, these instructions can only encode eight DALU registers (D0–D7) and