The Thumb instruction set is a subset of the ARM instruction set, designed to improve code density by using a 16-bit instruction width. Compared to equivalent 32-bit ARM instructions, Thumb reduces the memory footprint without sacrificing the performance benefits of the 32-bit architecture. However, it is not a complete architecture on its own—processors must be able to switch between Thumb and ARM modes to execute all types of instructions.
Thumb instructions are divided into several categories, including branch instructions, data transfer operations, single-register load/store instructions, and multi-register load/store instructions. Notably, Thumb does not support coprocessor instructions, semaphore operations, or direct access to the CPSR (Current Program Status Register) or SPSR (Saved Program Status Register).
One key difference between Thumb and ARM is that Thumb inherits many features from the ARM instruction set, such as the load-store architecture, which includes data processing, data movement, and control flow instructions. However, it also omits some advanced features found in ARM. For example, most Thumb instructions are executed unconditionally (except for the branch instruction), while all ARM instructions can be conditionally executed. Additionally, many Thumb data processing instructions use a two-address format, where the destination register is the same as one of the source registers, whereas most ARM instructions use a three-address format (except for 64-bit multiply instructions).
Another important distinction is related to exception handling. When an exception occurs, the processor switches to ARM mode to handle it, regardless of whether the code was running in Thumb mode. This is because ARM requires word-aligned addresses (divisible by 4), while Thumb uses half-word alignment (divisible by 2). As a result, the offset used when entering an exception differs between the two modes, which can affect how exceptions are handled in Thumb code.
Overall, the Thumb instruction set provides a more compact code size, making it ideal for applications with limited memory resources, while still maintaining compatibility with the full ARM instruction set through mode switching.
Armored Fiber Optic Cable, Fiber Optic Cable Manufacturers, Multimode Fiber Optic Cable, Single Mode Fiber Optic Cable
NINGBO YULIANG TELECOM MUNICATIONS EQUIPMENT CO.,LTD. , https://www.yltelecom.com