prfm

Prefetch Memory (Register)

PRFM <prfop>, [<Xn|SP>, <R><m> {, <extend> <amount>}]

Prefetches data using a register offset.

Details

Prefetch Memory (Register) signals the memory system to prefetch data from an address calculated by adding an optionally shifted register offset to a base register. The shift amount and extension type are encoded in the option and S bits. This is a hint instruction that does not affect register state or condition flags. Execution is AArch64-only.

Pseudocode Operation

offset ← ExtendValue(Rm, option, S); address ← Xn|SP + offset; Prefetch(address, prfop);

Example

PRFM prfop, [x1, Rm ]

Encoding

Binary Layout
11
111
0
00
10
1
Rm
option
S
10
Rn
Rt
 
Format Load/Store Reg
Opcode 0xF8A04800
Extension Base

Operands

  • prfop
    Type
  • Xn
    First source / base 64-bit integer register
  • Rm
    Second source / offset general-purpose register

Reference (Arm A64 ISA)

Instruction Forms

Encoding Instruction ISA Bit pattern
0xF9800000 PRFM (<prfop>|#<imm5>), [<Xn|SP>{, #<pimm>}] A64 11 | 111 | 0 | 01 | 10 | imm12 | Rn | Rt
0xD8000000 PRFM (<prfop>|#<imm5>), <label> A64 11 | 011 | 0 | 00 | imm19 | Rt
0xF8A04800 PRFM (<prfop>|#<imm5>), [<Xn|SP>, (<Wm>|<Xm>){, <extend> {<amount>}}] A64 11 | 111 | 0 | 00 | 10 | 1 | Rm | option | S | 10 | Rn | Rt

Description

Prefetch Memory (register) signals the memory system that data memory accesses from a specified address are likely to occur in the near future. The memory system can respond by taking actions that are expected to speed up the memory accesses when they do occur, such as preloading the cache line containing the specified address into one or more caches. The effect of a PRFM instruction is IMPLEMENTATION DEFINED. For more information, see Prefetch memory. For information about memory accesses, see Load/Store addressing modes.

Operation

bits(64) offset = ExtendReg(m, extend_type, shift, 64);
bits(64) address;

boolean privileged = PSTATE.EL != EL0;
AccessDescriptor accdesc = CreateAccDescGPR(MemOp_PREFETCH, FALSE, privileged, FALSE);

if n == 31 then
    address = SP[];
else
    address = X[n, 64];

address = GenerateAddress(address, offset, accdesc);

Prefetch(address, t<4:0>);