Introduction to ARM

Registers

ARM has sixteen registers visible at any one time. They are named R0 to R15. All are 32 bits wide.

Diagram of registers.

The registers may also be known by the following aliases†:

Diagram of registers' aliases.

All of the registers are general purpose, save for:

R13 / SP
which holds the stack pointer. ‡
R14 / LR
the link register which holds the callers’s return address.
R15 / PC
which holds the program counter.

In addition to the main registers there is also a status register:

Diagram of status registers.

CPSR is the current program status register. This holds flags: results of aritmetic and logical operations.

Load-Store Architecture

ARM is a load-store architecture:

  • You must load values into registers in order to operate upon them.
  • No instructions directly operate on values in memory.

Remarks

This slide shows the user mode register set.

† Register aliases are subject to the current calling convention. The ones shown here are apply to AAPCS – the ARM standard calling convention.

‡ Strictly R13 is used for the stack pointer by convention, rather than a rule, in ARM mode. In Thumb mode it is more hardwired.

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