HP 3000 III Series Manual page 42

System/CPU Overview
DB
----r-----------"
GLOBAL VARIABLES DB· plus dIrect relatIve
addressing (up to
DB+255)
LOCAL ARRAYS
GLOBAL ARRAYS
LOCAL VARIABLES
TEMPORARY VARIABLES &
INTERMEDIATE RESULTS
}
DB . plus IIldlrect relative
addressing
~=============~
I
PARAMETERS
I)
0
mlllus relative
addressing (up to
0 . 63)
Q
----:==============~
}
O·
plus direct relative
addressing (up to
0+127)
~==============~
}
O·
plus Indirect relatIve
addresslllg
~==================~
}
S·
minus relative
addressing (up to
S . 63)
~::;:::;=;::::::;::;:::::;::::;::::=;::;:::;~
Figure 2-3. Typical Data Stack
The data in the
DB
location is the oldest element on the stack.
The data in the S-pointer location is the most current element on
the stack. (The S-pointer location is referred to as the Top of
the Stack (TOS). Conventionally, TOS is represented downward
fram DB to correspond to the normal progression of writing soft-
ware programs where the most recently written statement is fur-
ther down the page than previously written statements. The area
fram S+l to Z is available for adding elements to the stack.
When a data word is added to the stack, it is stored in the next
available location and and the S-pointer is automatically incre-
mented by one to reflect the new TOS. (This process is said to
"push" a word onto the stack.) When data is deleted from the
stack, the S-pointer is decremented which puts the deleted word
in an undefined area. S-minus relative addressing is used to
refer to recently stacked elements of data and is one of the
standard addressing conventions. Under this convention, S-l is
the second element on the stack, 8-2 is the third element on the
stack, etc. The other standard addressing conventions are
DB-plus relative addressing, Q-rninus relative addressing, and
2-10