ARM: Introduction to ARM: Thumb

by David Thomas on

Thumb

Thumb is a subset of the ARM instruction set encoded in 16-bit wide instructions.

  • Requires 70% of the space of ARM code.
  • Uses 40% more instructions than equivalent ARM code.

A CPU has Thumb support if it has a T in its name, or it is architecture v6 or later.

  • With 32-bit memory:
    • ARM code is 40% faster than Thumb code.
  • With 16-bit memory:
    • Thumb code is 45% faster than ARM code.

Uses 30% less external memory power than ARM code.

Remarks

The slide quotes ARM’s “ideal” values. Take with a pinch of salt.

Limitations

Thumb is not a complete architecture: you can’t have a Thumb-only CPU.

Some of the limitations of Thumb mode include:

  • Conditional execution only exists for branch instructions.
  • Data processing ops use a two-address format, as opposed to ARM’s three-address format.
  • Its instruction encodings are less regular than ARM’s.

Thumb uses the same register set as ARM — but only R0-R7.

Remarks

Whereas R13’s role as the stack pointer is a convention in ARM mode, in Thumb mode it’s more hard-wired.

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