One-Word Low Register Prefix; Conditional Execution - Freescale Semiconductor SC140 DSP Core Reference Manual

5.2.2.2 One-Word Low Register Prefix

The One-Word Low register prefix encodes all information of the two-word prefix, except for encoding for
high registers. It is used whenever no instruction in the VLES uses a high register, and a prefix is needed
for one of the other reasons: instruction grouping that cannot be done with serial grouping, conditional
execution of the whole VLES or a sub-group, and encoding for HW loop support.

5.2.3 Conditional Execution

Certain instructions are executed conditional on the state of the T-bit in the status register (SR). For
example, JT (Jump if True) is executed only if the T-bit is set. The SC140 also supports conditional
execution of a group or subgroup of instructions in an execution set. A group or subgroup represents the
instructions that may be conditionally executed depending upon a single condition encoded in the prefix.
The single condition is specified by the following prefix instructions:
Instruction
IFT (if true)
IFF (if false)
IFA (if always)
For example:
IFT ADD D0,D1,D2
The set as a whole (including the ADD and MOVE instructions) is executed only if the T-bit is set. The
instructions in the subgroups may themselves be conditional. For example, using MOVEc, TFRc and Jc
can add further conditional control. However, the subgroups themselves may not contain another IFc
instruction.
If no IFc instructions exist in the execution set, the default is the unconditional execution of the whole set.
For finer control, it is possible to split the instructions in the execution set into two subgroups 1 and 2,
conditionally controlling the execution of each subgroup independently. Refer to
"Instruction Reordering Within an Execution Set,"
instructions for encoding conditional execution sets.
Table 5-4 displays the conditional IFc syntax. In the table, [inst] represents optional additional instructions.
Refer to Section 7.5.3, "Prefix Grouping Rules,"
Assembly Syntax
[IFA] inst [inst]
SC140 DSP Core Reference Manual
Table 5-3. Prefix Instructions
Execute the group or subgroup if the T bit is set
Execute the group or subgroup if the T bit is clear
Execute the group or subgroup unconditionally
MOVE.L (R0)+,D0
for information about how the assembler reorders
on page 7-11 for the IFc programming rules.
Table 5-4. Conditional IFc Syntax
Meaning
Unconditional execution of the VLES
Description
Section 5.2.5,
Instruction Grouping
5-9