Accessing Db- Area - HP 3000 III Series Manual

WORD
ADDRESSING
LOAD DB+4, I
or----+---
.,
08-10 ~ -I
DB
~
-I
08+4 177770
.H
BYTE
ADDRESSING
LDB DB+4, I
System/CPU Overview
0....--+----
Dl---t--+-_ _
---4
,r
08-4 -10 -7
-6 -5
-4 -3
- 2 -1
DB--+ 0
+1
08+4 177770
Address Calculations
In
Octal: WORD DB
+
177770 ~ DB - 10
BYTE DB
+
(177770 -:- 2)
+
100000 = DB - 10
Figure 2-18. Accessing DB- Area
will add 32K (%100000) to the DB+77774 value. Assuming that
wrap-around has not yet occured, this addition will cause it to
occur and thus address the byte at byte address DB-IO (left byte
in location DB-4). At this time, a second test is made to check
if the effective address is within the DL to Z range. If the
technique has been applied properly, the test will be affirmative
and the byte will be transferred. If the test fails during user
mode, there will be a bounds violation interrupt. If the test
fails during privileged mode, the test results will be ignored
and execution will continue (even if out of bounds), using the
second referenced byte.
2-65.
Bounds Checking
The CPU routinely checks all address references and TOS movements
to ensure that such operations remain within legal bounds. Suf-
ficient checks are made for all machine instructions to ensure
that a nonprivileged user cannot adversely affect other users or
the operating system. The basic bounds checks that are made for
the applicable instruction types are discussed in paragraphs 2-66
through 2-70 and summarized in table 2-6. The boundry limits
checked are illustrated in figure 2-19. If any of the bounds
check fail during non-privileged user mode, there will be a
bounds violation interrupt. Those checks whose results are ig-
nored during privileged mode are so indicated.
2-43