MSUBPT

Multiply-subtract checked pointer

This instruction multiplies two register values, subtracts the product from a third register value, and writes the result to the destination register. The intermediate product is treated as the offset.

If the operation would have generated a result where the most significant 8 bits of the result register differ from the most significant 8 bits of the base register, then the result is modified such that it is likely to be non-canonical when used as an address.

If the intermediate product cannot be correctly represented as a 64-bit two's complement value, then the result is modified such that it is likely to be non-canonical when used as an address.

Encoding: Integer

Variants: FEAT_CPA (ARMv9.5)

313029282726252423222120191817161514131211109876543210
100110110111
sfop54op31Rmo0RaRnRd

MSUBPT <Xd>, <Xn>, <Xm>, <Xa>

Decoding algorithm

if !IsFeatureImplemented(FEAT_CPA) then EndOfDecode(Decode_UNDEF);
constant integer d = UInt(Rd);
constant integer n = UInt(Rn);
constant integer m = UInt(Rm);
constant integer a = UInt(Ra);

Operation

constant bits(64) operand1 = X[n, 64];
constant bits(64) operand2 = X[m, 64];
constant bits(64) operand3 = X[a, 64];

bits(64) result;

constant integer product = SInt(operand1) * SInt(operand2);

// Signed and unsigned twos complement arithmetic are equivalent if only a
// fixed number of bits are considered.
result = operand3 - product<63:0>;

constant boolean overflow = (product != SInt(product<63:0>));
result = PointerMultiplyAddCheck(result, operand3, overflow);

X[d, 64] = result;

Explanations

<Xd>: Is the 64-bit name of the general-purpose destination register, encoded in the "Rd" field.
<Xn>: Is the 64-bit name of the first general-purpose source register holding the multiplicand, encoded in the "Rn" field.
<Xm>: Is the 64-bit name of the second general-purpose source register holding the multiplier, encoded in the "Rm" field.
<Xa>: Is the 64-bit name of the third general-purpose source register holding the minuend, encoded in the "Ra" field.