3.5.17. UASX and USAX

Unsigned Add and Subtract with Exchange and Unsigned Subtract and Add with Exchange.

Syntax

op{cond} {Rd,} Rn, Rm

Where:

op

Is one of:

UASX

Add and Subtract with Exchange.

USAX

Subtract and Add with Exchange.

cond

Is an optional condition code. See Conditional execution.

Rd

Is the destination register. If Rd is omitted, the destination register is Rn.

Rn

Is the first operand register.

Rm

Is the second operand register.

Operation

The UASX instruction:

  1. Subtracts the top halfword of the second operand from the bottom halfword of the first operand.

  2. Writes the unsigned result from the subtraction to the bottom halfword of the destination register.

  3. Adds the top halfword of the first operand with the bottom halfword of the second operand.

  4. Writes the unsigned result of the addition to the top halfword of the destination register.

The USAX instruction:

  1. Adds the bottom halfword of the first operand with the top halfword of the second operand.

  2. Writes the unsigned result of the addition to the bottom halfword of the destination register.

  3. Subtracts the bottom halfword of the second operand from the top halfword of the first operand.

  4. Writes the unsigned result from the subtraction to the top halfword of the destination register.

Restrictions

Do not use SP and do not use PC.

Condition flags

These instructions set the APSR.GE bits according to the results.

For UASX:

if ConditionPassed() then
   EncodingSpecificOperations();
   diff = UInt(R[n]<15:0>) - UInt(R[m]<31:16>);
   sum = UInt(R[n]<31:16>) + UInt(R[m]<15:0>);
   R[d]<15:0> = diff<15:0>;
   R[d]<31:16> = sum<15:0>;
   APSR.GE<1:0> = if diff >= 0 then '11' else '00';
   APSR.GE<3:2> = if sum >= 0x10000 then '11' else '00';

For USAX:

if ConditionPassed() then
   EncodingSpecificOperations();
   sum = UInt(R[n]<15:0>) + UInt(R[m]<31:16>);
   diff = UInt(R[n]<31:16>) - UInt(R[m]<15:0>);
   R[d]<15:0> = sum<15:0>;
   R[d]<31:16> = diff<15:0>;
   APSR.GE<1:0> = if sum >= 0x10000 then '11' else '00';
   APSR.GE<3:2> = if diff >= 0 then '11' else '00';

Examples

UASX    R0, R4, R5     ; Adds top halfword of R4 to bottom halfword of R5 and 
                       ; writes to top halfword of R0.
                       ; Subtracts bottom halfword of R5 from top halfword of R0
                       ; and writes to bottom halfword of R0.
USAX    R7, R3, R2     ; Subtracts top halfword of R2 from bottom halfword of R3 
                       ; and writes to bottom halfword of R7.
                       ; Adds top halfword of R3 to bottom halfword of R2 and 
                       ; writes to top halfword of R7.
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