Editing Byte addressing (section)

==Hybrid systems==
Some systems with [[word addressing]], such as the [[PDP-10|PDP-6/10]] and the [[GE-600 series|GE-600]]/[[Honeywell 6000 series|Honeywell 6000]] series, have special mechanisms for accessing bytes efficiently.

On the PDP-6/10, special instructions operated on a ''byte pointer'' which included a word address, a bit offset, and a bit width. The {{Mono|LDB}}/{{Mono|DPB}} instructions loaded or stored one byte, the {{Mono|IBP}} instruction incremented the byte pointer, and the {{Mono|ILDB}}/{{Mono|IDPB}} instructions incremented the byte pointer and then loaded or stored the next byte. These instructions could operate on arbitrary-width bit fields.<ref name="1982-pdp10-proc-ref-man">{{cite book|url=http://bitsavers.org/pdf/dec/pdp10/1982_ProcRefMan.pdf|title=DECsystem-10/DECSYSTEM-20 Processor Reference Manual|id=AD-H391A-T1|date=June 1982|publisher=[[Digital Equipment Corporation]]}}</ref>{{rp|pages=2{{hyp}}85-2{{hyp}}89}} Programs took advantage of this flexibility: those not needing lowercase letters used the limited character set of [[Six-bit character code|6-bit bytes]] for efficiency; most used [[7-bit ASCII]], packed 5 to a word with one unused bit; and the [[C (programming language)|C]] implementation used 9-bit bytes because C requires all memory to be byte-addressable.

On the GE/Honeywell machines, special [[Addressing mode|indirect addressing modes]] could be used on most instruction types, and operated on a byte pointer which could operate on either 6-bit or 9-bit bytes.<ref>{{cite book|url=http://www.bitsavers.org/pdf/ge/GE-6xx/CPB-1004F_GE-635_Prog_Ref_Man_196907.pdf|title=GE-625/635 Programming Reference Manual|date=July 1969|publisher=[[General Electric]]|pages=169, 171–172}}</ref>

Neither of these machines originally had direct machine support for random access to bytes; adjusting a byte pointer to point ''N'' bytes before or after the byte to which it currently pointed required a sequence of multiple instructions.  The KL10 PDP-10 model extended the {{Mono|IBP}} instruction to become the "adjust byte pointer" instruction, {{Mono|ADJBP}}, that could adjust a byte pointer by an arbitrary number of bytes.<ref name="1982-pdp10-proc-ref-man" />{{rp|pages=2{{hyp}}89-2{{hyp}}91}}