Hi, Branden wrote: Before RISC-V, the need for code density caused ARM to move away from the original, highly regular instruction format of one 32-bit word per instruction to Thumb and then Thumb-2 encoding. IIRC, Thumb moved to 16-bit per instruction which was expanded by Thumb-2 to also have some 32-bit instructions. The mobile market was getting going and the storage options and their financial and power costs meant code density mattered more. The original ARM instructions had the top four bits hold the ‘condition code’ which decided if the instruction was executed, thus the top hex nibble was readable. 0 f eq ne cs cc mi pl vs vc hi ls ge lt gt le al nv Data processing instructions, like and rd, rn, rm ; d = n & m aligned each of the four-bits to identify which of the sixteen registers were used on nibble boundaries so again it was readable as hex. xxxx000a aaaSnnnn ddddcccc ctttmmmm The ‘a aaa’ above wasn't aligned, but still neatly picked which of the sixteen data-processing instructions was used. and eor sub rsb add adc sbc rsc tst teq cmp cmn orr mov bic mvn And so it went on. A SoftWare Interrupt had an aligned 1111 to select it and the low twenty-four bits as the interrupt number. xxxx1111 yyyyyyyy yyyyyyyy yyyyyyyy I assume this neat arrangement helped keep the decoding circuitry small leading to a simpler design and lower power consumption. The latter was important because Acorn, the ARM chip's designer, wanted a cheaper plastic case rather than ceramic so they set a design limit of 1 W. Due to the poor tooling available, it came in at 0.1 W after allowing for a margin of error. This was so low that Acorn were surprised when an early board ran without power connected to the ARM; they found it was clocking just from the leakage of the surrounding support chips. Also, Acorn's Roger Wilson who designed the ARM's instruction set was an expert assembly programmer, e.g. he wrote the 16 KiB BASIC ROM for 6502, so he approached it from the programmer's viewpoint as well as the chip designer he became. Thumb and Thumb-2 naturally had to destroy all this so instructions are now not orthogonal. Having coded swtch() in assembler for various ARM Cortex M-..., it's a pain to have to keep checking what instructions are available on this model and what registers can it access. On ARM 2, there were few rules to remember and writing assembler was fun. -- Cheers, Ralph.