The 32 integer registers and their names

What a register is

A processor needs somewhere close by to keep the values it is working with. A register is one of these small storage places inside the processor. There are far fewer registers than locations in memory, but the processor can work with registers directly and quickly.

The basic 32-bit RISC-V design has 32 integer registers. Each one holds 32 bits: 32 binary digits that can each be zero or one. A program can use that pattern of bits as a number, an address or another kind of value.

Registers already exist and already have names. When writing assembly, you choose which register will hold each value and keep track of that choice.

Numbered names and aliases

The 32 integer registers have numbered names from x0 through x31. RISC-V assembly also gives them descriptive names such as t0, s0 and a0.

These are two naming systems for the same set of registers. For example:

numbered name	descriptive name
x0	zero
x5	t0
x8	s0 or fp
x10	a0

An alternative name for the same thing is called an alias. x5 and t0 name one storage place. If that register holds 12, reading either x5 or t0 gives 12. Replacing the value through the name t0 also replaces the value seen through the name x5.

The same applies to x8, which has two descriptive aliases. x8, s0 and fp are three accepted names for one register.

The descriptive names are often called the ABI names. An ABI is a set of conventions that helps separately written parts of a program work together. The numbered name tells you which register it is; the ABI name also suggests the job that register usually has.

The complete register table

Use this table as a lookup when you meet an unfamiliar register name. You do not need to memorize it.

numbered name	ABI name(s)	conventional role
x0	zero	always reads as zero
x1	ra	return address
x2	sp	stack pointer
x3	gp	global pointer
x4	tp	thread pointer
x5-x7	t0-t2	temporary values
x8	s0 / fp	saved value; also an optional frame pointer
x9	s1	saved value
x10-x11	a0-a1	arguments and return values
x12-x17	a2-a7	arguments
x18-x27	s2-s11	saved values
x28-x31	t3-t6	temporary values

The table gives each ABI name its conventional meaning. t0 through t6 are temporary registers: programs commonly use them for values needed during a piece of work. Keep the table nearby when you encounter the other names.

With one exception, these roles are agreements between programmers and tools. The processor allows any ordinary register to hold a value. The group names make their conventional jobs visible when you read a program.

The register that is always zero

x0, also called zero, is the exception. Its behaviour is part of the processor design, not just a naming convention:

• Reading zero always gives 0.
• Trying to replace its value has no effect. The new value is discarded.

For example, imagine that a program tries to place 99 in x0. Reading either x0 or zero afterwards still gives 0. A program can use this guaranteed zero as an input or discard a result by placing it in zero.

Do not assume that the other registers begin at zero. Their starting values depend on the system running the program. Only x0 is guaranteed by RISC-V to read as zero at all times.

Check the names

Use the table to answer these questions:

1. If t0 holds 37, what value does x5 hold?
2. A value is placed in x8. Which two descriptive names refer to that same value?
3. What are the descriptive names for x18 and x19?
4. A program tries to replace the value in zero with 8. What will the next read from x0 produce?

Show answers

1. x5 holds 37 because x5 and t0 are aliases for one register.
2. s0 and fp both refer to x8.
3. x18 is s2, and x19 is s3.
4. It produces 0. A value written to zero is discarded.

The main ideas are simple: a register is a small storage place inside the processor, each integer register has an x number, and its ABI name is an alias that suggests its conventional role. zero is special because the processor guarantees that it always reads as 0.