What is the difference between ARM and Thumb mode?

What is the difference between ARM and Thumb mode?

ARM and Thumb are two different instruction sets supported by ARM cores with a “T” in their name. For instance, ARM7 TDMI supports Thumb mode. ARM instructions are 32 bits wide, and Thumb instructions are 16 wide. Thumb mode allows for code to be smaller, and can potentially be faster if the target has slow memory.

What is Thumb mode ARM?

The Thumb instruction set consists of 16-bit instructions that act as a compact shorthand for a subset of the 32-bit instructions of the standard ARM. When it’s operating in the Thumb state , the processor simply expands the smaller shorthand instructions fetched from memory into their 32-bit equivalents.

What is Thumb instruction set what is the difference between thumb 1 and thumb 2?

Advanced Member level 1 Thumb = 16 bit instructions. Better performance / bigger code size in ARM mode vs Thumb. Thumb 2 is mixed 32/16 bit instructions. Somewhere in between the two.

What is Thumb code?

Thumb code is typically 65% of the size of ARM code, and provides 160% of the performance of ARM code when running from a 16-bit memory system. Thumb, therefore, makes the ARM7TDMI core ideally suited to embedded applications with restricted memory bandwidth, where code density and footprint is important.

What is Thumb state?

A processor that is executing Thumb instructions is said to be operating in Thumb state. A Thumb-capable processor that is executing ARM instructions is said to be operating in ARM state. ARM processors always start in ARM state.

What’s the alignment requirement for Thumb instructions?

ARM instructions have fixed-width 4-byte encodings which require 4-byte alignment. Thumb instructions have variable-length (2 or 4-byte, now known as “narrow” and “wide”) encodings requiring 2-byte alignment – most instructions have 2-byte encodings, but bl and blx have always had 4-byte encodings*.

How is switching between ARM and Thumb state done?

These directives do not change the instruction set state of the processor. To do this, you must use an appropriate instruction, for example BX or BLX to change between ARM and Thumb states when performing a branch, or ENTERX and LEAVEX to switch between Thumb and ThumbEE states.

How do I switch to Thumb mode?

To change between ARM and Thumb states, you must switch the assembler mode to produce the correct opcodes using ARM or THUMB directives. To generate ThumbEE code, use THUMBX . Assembler code using CODE32 and CODE16 can still be assembled by the assembler, but you are recommended to use ARM and THUMB for new code.