Hacker News Reader

Sounds like M68K or something similar, although Alpha AXP had similar byte level access issues. A compiler on either of those platforms likely would add a lot of fix up code to deal with the fact they have to load the aligned (either 16bit in M68K case or 32Bit IIRC in Alpha) and then do bitwise and shifts depending on the pointers lower bits.

Raymond's blog on the Alpha https://devblogs.microsoft.com/oldnewthing/20170816-00/?p=96...

View on HN

M68k was byte addressable just fine. Early alpha had that issue though, as did later cray compilers. Alpha fixed it with BWX (byte word extension). Early cray compilers simply defined char as being 64bits, but later added support for the shift/mask/thick pointer scheme to pack 8 chars in a word.

Must have depended on variant, the one we used in college would throw a GP fault for misaligned access. It literally didn't have an A0 line. That said it's been over 10 years and I could be remembering the very hard instruction alignment rules as applying to data too...

16bits had to be aligned. It didn't have an A0 because of the 16bit pathway, but it did have byte select lines (#UDS, #LDS) for when you'd move.b d0,ADDR so that devices external to the CPU could see an 8-bit data access if that's what you were doing.

Reply on news.ycombinator.com