9.3. Arithmetic Instructions

9.3.1. Common Arithmetic Instructions

The A64 ISA implements several instructions that correspond with arithmetic operations performed by the ALU. Table 1 lists several arithmetic instructions that one may encounter when reading ARM assembly:

Table 1. Common instructions.
Instruction Translation

add D, O1, O2

D = O1 + O2

sub D, O1, O2

D = O1 - O2

neg D, O1

D = -(O1)

The add and sub instructions correspond to addition and subtraction and require two operands in addition to the destination register. In contrast, the neg instruction requires only one operand in addition to the destination register.

The three instructions shown above also have carry forms that enable the instruction to use the optional carry condition flag, C. The one-bit carry flag is set when an unsigned operation overflows. We cover other condition control flags in the following section, but introduce the carry flag here to introduce the additional arithmetic instructions. The carry forms and their rough translation are shown in Table 2.

Table 2. Carry forms for common instructions.
Instruction Translation

adc D, O1, O2

D = O1 + O2 + C

sbc D, O1, O2

D = O1 - O2 - ~C

ngc D, O1

D = -(O1) - ~C

The above instructions also have an optional s suffix. When the s suffix is used (e.g. adds), it indicates that the arithmetic operation is setting condition flags.

Multiplication and Division

Table 3. Common multiplication and division instructions.
Instruction Translation

mul D, O1, O2

D = O1 * O2

udiv D, O1, O2

D = O1 / O2 (32-bit unsigned)

sdiv D, O1, O2

D = O1 / O2 (64-bit signed)

The most common multiplication and division instructions are shown in Table 3. The mul instruction operates on two operands and places the product in the destination D. The division operation does not have a generic form; the udiv and sdiv instructions operate on 32-bit and 64-bit data respectively. Note that you cannot multiply 32-bit registers with 64 bit registers.

In addition, ARMv8-A provides composite forms for multiplication, allowing the CPU to perform more sophisticated operations in a single instruction. These instructions are shown in Table 4:

Table 4. Composite multiplication instructions.
Instruction Translation

madd D, O1, O2, O3

D = O3 + (O1 * O2)

msub D, O1, O2, O3

D = O3 - (O1 * O2)

mneg D, O1, O2

D = -(O1 * S2)

9.3.2. Bit Shifting Instructions

Bit shifting instructions enable the compiler to perform bit shifting operations. Multiplication and division instructions typically take a long time to execute. Bit shifting offers the compiler a shortcut for multiplicands and divisors that are powers of 2. For example, to compute 77 * 4, most compilers will translate this operation to 77 << 2 to avoid the use of an mul instruction. Likewise, to compute 77 / 4, a compiler typically translates this operation to 77 >> 2 to avoid using the sdiv instruction.

Keep in mind that left and right bit shift translate to different instructions based on whether the goal is an arithmetic (signed) or logical (unsigned) shift:

Table 5. Bit Shift Instructions
Instruction Translation Arithmetic or Logical?

lsl D, R, #v

D = R << v

logical or arithmetic

lsr D, R, #v

D = R >> v

logical

asr D, R, #v

D = R >> v

arithmetic

ror D, R, #v

D = R >>> v

neither (rotate)

In addition to the destination register, each shift instruction takes two operands, one which is usually a register (denoted by R) and the other which is a 6-bit shift value (v). On 64-bit systems, the shift value is encoded as a single byte (since it doesn’t make sense to shift past 63). The shift value v must either be a constant or stored in a component register.

The last bit shifting instruction, ror, requires special discussion. The ror instruction rotates the bits, replacing the most significant bits with the least significant bits. We represent the rotate shift instruction using the >>> symbol.

Different versions of instructions help us distinguish types at an assembly level

At the assembly level, there is no notion of types. However, recall that the compiler can choose to use component registers based on the types present at the code level. Similarly, recall that shift right works differently depending on whether or not the value is signed or unsigned. At the assembly level, the compiler uses separate instructions to distinguish between logical and arithmetic shifts!

9.3.3. Bitwise Instructions

Bitwise instructions enable the compiler to perform bitwise operations on data. One way the compiler uses bitwise operations is for certain optimizations. For example, a compiler may choose to implement 77 mod 4 with the operation 77 & 3 in lieu of the more expensive sdiv instruction.

The table below lists common bitwise instructions, and composite bitwise instructions that utilize negation:

Table 6. Bitwise Operations
Instruction Translation

and D, O1, O2

D = O1 & O2

orr D, O1, O2

D = O1 | O2

eor D, O1, O2

D = O1 ^ O2

mvn D, O

D = ~O

bic D, O1, O2

D = O1 & ~O2

orn D, O1, O2

D = O1 | ~O2

eon D, O1, O2

D = O1 ^ ~O2

Remember that bitwise not is distinct from negation (neg). The mvn instruction flips the bits of the operand but does not add 1. Be careful not to confuse these two instructions.

Use bit operations only when needed in your C code!

After reading this section, it may be tempting to replace common arithmetic operations in your C code with bitwise shifts and other operations. This is NOT recommended. Most modern compilers are smart enough to replace simple arithmetic operations with bitwise operations when it makes sense, making it unnecessary for the programmer to do so. As a general rule, programmers should prioritize code readability whenever possible and avoid premature optimization.