On 2009-01-09, Dennis Lee Bieber <wlfr...@ix.netcom.com> wrote: > On Thu, 08 Jan 2009 09:46:26 -0600, Grant Edwards <inva...@invalid> > declaimed the following in comp.lang.python: > > >> Heathkit Z80 stuff used octal notation too. > > Octal worked well for the old 8080 and derivative processors as > there were only 7 registers and "memory/indirect" to encode in an > instruction... So (pseudo example) a MOV command might have been > > 01sssddd (binary) > > or > > 1SD (octal) > > > and S or D representing > > A 1 > B 2 > C 3 > D 4 > E 5 > H 6 > L 7 > Mem 0 (indirect via contents of HL pair)
I presume that's why DEC chose octal for the PDP-11 also. There were 8 registers and 8 addressing modes, so they ended up with several fields within the opcodes that were three-bits wide and aligned with octal digits: http://en.wikipedia.org/wiki/PDP-11#Instruction_set When I was in University, we spent an entire quarter studying the PDP-11 instruction set at the binary/octal level and discussing instruction decoding and sequencing. We wrote a few PDP-11 assembly language programs as well, but the course was more about machine level instruction sets than about assembly language. Octal made a lot of sense if you had to deal with raw memory, but once you had access to an assembler and disassembler it didn't really matter. -- Grant Edwards grante Yow! NANCY!! Why is at everything RED?! visi.com -- http://mail.python.org/mailman/listinfo/python-list
