Byte Addressing Examples - HP 3000 III Series Manual

System/CPU Overview
DIRECT
STB OB+7
DIRECT, INDEXED
LOXI5
STB OB+7, X
INDIRECT
STB OB+7, I
INDIRECT, INDEXED
LOXI 5
STB OB+7,I,X
DB
s
---..
t1
+2
Not
+3
+4
Access-
Ible
+5
;-6
+7
---..
DB
s
--.
+1
+2
+3
+4
;-5
;-6
+7
0 1
2 3
4 5
---.
DB
s
---..
+1
+2
+3
+4
;-5
;-6
;-7
46
+20 40 41
+21
42
43
+22 44 45
+23
46 47
---..
DB
s
~
+1
+2
+3
+4
+5
+6
+7 40
+20 0 1
+21 2 3
+22 4
5
---..
Figure 2-17. Byte Addressing Examples
tion would store a byte from the TOS into the left byte of the
DB+7 loca tion.
2-58. DIRECT, INDEXED BYTE ADDRESSING. In the examples shown in
figure 2-17, the index is assumed to be 5. This is established
by the "LDXI5" instruction that precedes each STB instruction.
The "STB DB+7, X" instruction directly addresses location DB+7
and the index of 5 accesses the sixth byte. It should be noted
that the byte index starts at zero and that all even indexes are
left bytes and all odd indexes are right bytes.
2-59. INDIRECT BYTE ADDRESSING. For indirect, unindexed byte
addressing, the byte index is given in the indirect cell. As in
all indirect data addressing, the indirect reference is relative
to DB. Therefore, the "STB DB+7, I" instruction shown in figure
2-17 initially addresses the 47th byte in respect to DB. This
will be the left byte of DB+23. (Since there are two bytes per
word, divide the byte index by two to identify the word location;
a remainder of zero indicates the left byte and a remainder of
one ind ica tes the right byte.)
2-60. INDIRECT, INDEXED BYTE ADDRESSING. For indirect, indexed
byte addressing, the displacement points to the indirect cell,
the ind irect cell points to the star t of the byte ar ray, and the
index in the X Register points to the desired byte in the array.
This is shown by the "STB DB+7, I, X" instruction in figure 2-17.
The index in the X Registe r is aga in assumed to be 5. The dis-
2-41