isel

Integer Select

isel RT, RA, RB, BC

Conditionally copies RA or RB to RT based on a CR bit. (Equivalent to C ternary operator 'cond ? a : b').

Encoding

Binary Layout
31
0:5
RT
6:10
RA
11:15
RB
16:20
BC
21:25
15
26:30
/
31
 
Format A-form
Opcode 0x7C00001E
Extension Base

Operands

  • RT
    Target Register
  • RA
    Source (If True) - 0 means 0
  • RB
    Source (If False)
  • BC
    CR Bit Index (Condition)
  • CR
    Condition Register Field

Example

isel r3, r4, r5, 2

// r3 = (CR.eq) ? r4 : r5

Registers Altered

CR0, CR1-CR7

Related

Across architectures

Conditional Select (Branchless Move) : how x86, ARM, RISC-V, and PowerISA each do this.

More in Base

Reference

View in PowerISA v3.1C Specification ↗ OPF Power ISA v3.1C

Description

Conditionally selects between RA and RB based on the value of a single bit in the Condition Register, writing the result to RT. If the CR bit BC is 1, RT ← RA (or 0 if RA=0); otherwise RT ← RB. This provides a branch-free ternary operation.

Operation

BIT ← CR[BC]
if BIT = 1 then
  RT ← (RA)
else
  RT ← (RB)

Extended Mnemonics

Extended Mnemonic Equivalent Instruction
iselgt
iseleq
isellt

Programming Note

isel is used for conditional moves based on the condition register.