6 Mar
2025
6 Mar
'25
11:42 a.m.
From: Phil Budne
BUT, the basic TCP and IP protocols seem to have been
created with a
general care that two byte fields should be aligned at multiples of two
bytes
Yes, because dealing with a 16-bit field that spans two PDP-11 16-bit words
is a pain (espcially because the PDP-11 does not have a 'load byte into
register _without_ extending the sign bit into the high half' instruction).
Do realize that in addition to the early TCP implementation, the _first_ TCP
router (at that stage, TCP and IP were not separate protocols) was also a
PDP-11 (albeit programmed in BCPL, not MACRO-11).
I remember the extension being a real PITA. To load an un-aligned 16-bit
quantity into R0, one would have had to do something like (assuming a pointer
to the un-aligned 16-bit quantity was in R1):
MOVB (R1)+, R0
SWAB R0
BIC #0377, R0
BISB (R1)+, R0
There may have been a better way to do it, but that's the best I can come up
with now; I recall we had to do something like that.
Yes, the 16-bit fields were 16-bit word aligned.
Noel