Arm® A64 Instruction Set Architecture: Future Architecture Technologies in the A architecture profile

DECB, DECD, DECH, DECW (scalar)

Decrement scalar by multiple of predicate constraint element count.

Determines the number of active elements implied by the named predicate constraint, multiplies that by an immediate in the range 1 to 16 inclusive, and then uses the result to decrement the scalar destination.

The named predicate constraint limits the number of active elements in a single predicate to:

* A fixed number (VL1 to VL256)

* The largest power of two (POW2)

* The largest multiple of three or four (MUL3 or MUL4)

* All available, implicitly a multiple of two (ALL).

Unspecified or out of range constraint encodings generate an empty predicate or zero element count rather than Undefined Instruction exception.

It has encodings from 4 classes: Byte , Doubleword , Halfword and Word

Byte

if !HaveSVE() then UNDEFINED;
integer esize = 8;
integer dn = UInt(Rdn);
bits(5) pat = pattern;
integer imm = UInt(imm4) + 1;

Doubleword

if !HaveSVE() then UNDEFINED;
integer esize = 64;
integer dn = UInt(Rdn);
bits(5) pat = pattern;
integer imm = UInt(imm4) + 1;

Halfword

if !HaveSVE() then UNDEFINED;
integer esize = 16;
integer dn = UInt(Rdn);
bits(5) pat = pattern;
integer imm = UInt(imm4) + 1;

Word

if !HaveSVE() then UNDEFINED;
integer esize = 32;
integer dn = UInt(Rdn);
bits(5) pat = pattern;
integer imm = UInt(imm4) + 1;

Assembler Symbols

CheckSVEEnabled();
integer count = DecodePredCount(pat, esize);
bits(64) operand1 = X[dn];

X[dn] = operand1 - (count * imm);

Operational information

If PSTATE.DIT is 1:

• The execution time of this instruction is independent of:
  • The values of the data supplied in any of its registers.
  • The values of the NZCV flags.
• The response of this instruction to asynchronous exceptions does not vary based on:
  • The values of the data supplied in any of its registers.
  • The values of the NZCV flags.