27 Jun
2013
27 Jun
'13
10:37 a.m.
From: ben
Sent: Wednesday, June 26, 2013 7:17 PM
On 6/26/2013 7:53 PM, Guy Sotomayor wrote:
> On Jun 26, 2013, at 6:45 PM, ben <bfranchuk at
jetnet.ab.ca> wrote:
>> To bad they did not make the PDP-11 as 18 bits
rather than 16.
> Unibus is 18-bits of addressing. ;-)
I was thinking of a mini PDP 10 in terms of
Instructions.
Byte and Half word (signed) instructions only.
Remember, the PDP-10 is a word-addressed machine, not a byte-addressed
one.
*Instructions* are 36 bits long (9 op code, 4 AC, 1 indirect, 4 index,
18 address/immediate). Bytes are defined by 36-bit pointers which
define the length of the byte, the position of the byte within the
word, and the address of the word containing the byte. Halfword
instructions refer to the left or right halfword of the word addressed.
How do you fit that into an 18-bit machine based on an 8-bit byte?
(Hint: Look at DEC's 18-bit family to see what a real 18-bit system does.)
Rich
Rich Alderson
Vintage Computing Sr. Systems Engineer
Vulcan, Inc.
505 5th Avenue S, Suite 900
Seattle, WA 98104
mailto:RichA at vulcan.com
mailto:RichA at LivingComputerMuseum.org
http://www.LivingComputerMuseum.org/