Raspberry Pi Assembly Language Programming: ARM Processor Coding

© Stephen Smith 2019

S. SmithRaspberry Pi Assembly Language Programminghttps://doi.org/10.1007/978-1-4842-5287-1_15

15. Thumb Code

Stephen Smith¹ 

(1)

Gibsons, BC, Canada

 

The Assembly code we’ve been developing produces compact code compared to high-level languages due to not needing a runtime and each instruction only taking 32 bits. However, in the early days of the ARM processor, there were a lot of complaints that this was too large. People used ARMs in small embeddable devices with very limited RAM and needed more compact programs. Others created systems with a 16-bit memory bus that allowed 64K of memory—tiny by today’s standards and took two memory cycles to load each 32-bit instruction slowing down the processor.

ARM took these concerns and ...