vtbx

Vector Table Extension

VTBX<c>.8 <Dd>, <list>, <Dm>

Inserts elements into a vector using a table lookup.

Details

Performs a table lookup and extension: for each byte index in Dm, looks up the corresponding byte in the table (1–4 registers starting at Vn) and inserts it into the corresponding position in Dd, leaving unmapped indices unchanged in Dd. Condition flags (N, Z, C, V) are unaffected. Execution is restricted to A32/T32 with NEON extension; out-of-range indices preserve the original element in Dd.

Pseudocode Operation

for i = 0 to 15 do
  index ← Dm.byte[i]
  if (index < (len + 1) * 16) then
    table_reg ← Vn + (index / 16)
    element_offset ← index mod 16
    Dd.byte[i] ← [table_reg].byte[element_offset]
  else
    Dd.byte[i] ← Dd.byte[i]

Example

VTBX.8 d0, {r0-r3}, d2

Encoding

Binary Layout
111100111
D
11
Vn
Vd
10
len
N
1
M
0
Vm
 
Format NEON Table
Opcode 0xF3B00840
Extension NEON (SIMD)

Operands

  • Dd
    Dest/Base
  • list
    Table
  • Dm
    Indices

Reference (Arm AArch32 ISA)

Instruction Forms

Encoding Instruction ISA Bit pattern
0xF3B00840 VTBX{<c>}{<q>}.8 <Dd>, <list>, <Dm> A32 111100111 | D | 11 | Vn | Vd | 10 | len | N | 1 | M | 0 | Vm
0xFFB00840 VTBX{<c>}{<q>}.8 <Dd>, <list>, <Dm> T32 111111111 | D | 11 | Vn | Vd | 10 | len | N | 1 | M | 0 | Vm

Description

Vector Table Lookup uses byte indexes in a control vector to look up byte values in a table and generate a new vector. Indexes out of range return 0. Vector Table Extension works in the same way, except that indexes out of range leave the destination element unchanged. Depending on settings in the CPACR, NSACR, and HCPTR registers, and the Security state and PE mode in which the instruction is executed, an attempt to execute the instruction might be undefined, or trapped to Hyp mode. For more information see Enabling Advanced SIMD and floating-point support.

Operation

if ConditionPassed() then
    EncodingSpecificOperations();  CheckAdvSIMDEnabled();

    // Create 256-bit = 32-byte table variable, with zeros in entries that will not be used.
    table3 = if length == 4 then D[n+3] else Zeros(64);
    table2 = if length >= 3 then D[n+2] else Zeros(64);
    table1 = if length >= 2 then D[n+1] else Zeros(64);
    table = table3 : table2 : table1 : D[n];

    for i = 0 to 7
        index = UInt(Elem[D[m],i,8]);
        if index < 8*length then
            Elem[D[d],i,8] = Elem[table,index,8];
        else
            if is_vtbl then
                Elem[D[d],i,8] = Zeros(8);
            // else Elem[D[d],i,8] unchanged