Freescale Semiconductor SC140 specifications Serial Grouping, Prefix Grouping

Instruction Grouping

Prefix grouping can group together any instructions that have available execution units. However, the prefix method requires one additional instruction word per execution set. Serial grouping is more compact, but only supports a subset of instructions. The assembler automatically selects serial or prefix grouping based on the instructions in each execution set so that the encoding length is minimized. The grouping method selection algorithm is described in Figure 5-3.

5.2.1.1 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, on condition that a Type 2 instruction can be the last in VLES. Refer to Section 7.4.1.3, “Assembler Reordering,” on page 7-3.

•One Type 3 instruction grouped with up to five Type 1 instructions, on condition that a Type 3 instruction can be the last in VLES. Refer to Section 7.4.1.3, “Assembler Reordering,” on page 7-3.

•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.

5.2.1.2 Prefix Grouping

The SC140 architecture uses prefix words to encode architecture extensions for an entire execution set. The prefix word is a part of the execution set but is not issued directly to any of the execution units.

The grouping information encoded 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.

SC140 DSP Core Reference Manual 5-7