Hacker News Reader

Comment by laserdancepony | Hacker News Reader

The IA-32 ISA has been so successful _because_ of the no-compromise downward compatibility, whether you like that or not.

See all the revolutionary ISAs from the 80s, 90s and 00s that did away with all that legacy crap. Where are they now? I think the only one still working is POWER.

View on HN

The ARM architecture was introduced in 1985 and the first real shipping product, the Acorn Archimedes, in 1987.

ARM also is very backwards compatible, there have only been two major breaks in compatibility. The ARM6 (which dropped legacy memory addressing modes) and aarch64 (which made certain extensions required + dropped support for old ARM7 and ARM9 modes).

And the thumb stuff. You can buy 32-bit ARM chips that have zero binary instructions in common with any 32-bit ARM before 1994.

Yeah, sorry; thumb was part of the ARM7/9 features I was referring to. It also dropped the hardware Java support.

To be fair, both were always "optional" features; but yes, there are thousands of devices that shipped with them (one of the biggest being the GBA, with the majority of games heavily relying on THUMB).

The grandparent is referring to the fact the ARMv6-M and ARMv7-M architectures are Thumb only (and are different subsets of Thumb 1 and Thumb 2, to keep it spicy); so a Cortex M has zero instruction set overlap with a pre-Thumb ARM core, even though both are "32 bit ARM."

Or perhaps they have been successful primarily because of luck. They were lucky to be selected by IBM for the PC. Once they were riding the PC wave, their position as market leader (by volume, not quality) kept them there.

Reply on news.ycombinator.com