Download 6 - Forth Interest Group

both in function and code size. Assembly
code words which do not contain short
absolute addresses are also unaffected. For
those that do, their code has been modified
to use long absolute addressing. For the
programmer who is coding in assembly,
only two adjustments in coding technique
must be made. Follow each short absolute
address with the system's base-page address (BPAGE)and use the long absolute
assembly mnemonic [i.e., L#) vs. # ) 1
whenever addressing memory directly.
For example, to copy the 16-bit value
from the variable SPAN to D l , instead of
using:
SPAN
#)
MOVE
Dl
write:
SPAN BPAGE
L#)
Dl
MOVE
( $FF FFFF)
Where
F83 needs to be4
( RAM AREA )
-Top
of my system RAM
Memory grows
downwards
( I10 AREA )
r:SZrLram
P
( ROM AREA )
I
system Boot RoMs
Figure One. Memory map of the HP9920.
Page of Memory
m
-
Relative
Memory
Location
31
Address Register
15Forth addr
Page addr
4
A
System Memory
-4 64Kb page
Address of Memory Page
/
Figure Two. Representation of page and local addresses in memory.
Volwne IX.Number 6
21
0
where BPAGE is a 16-bit constant which
returns the 64K page address to which the
system was compiled. A second constant
BOFFSET specifies the starting address of
the kemel within this page. These two
words are defined twice in the kernel
source: Screen #1 contains the definitions
used by the metacompiler to target the kernel, and screen #I34 contains the definitions
for use by the Forth programmer.
To compile the kernel at a new target
address, simply edit the definitions BPAGE
and BOFFSET in screen #1 of the kemel,
and compile the system as described by
Laxen and Perry. (See Listing One, screen
#I.) Note: each listing contains the original
source screens in the left column, and its
corresponding 'extended' source screens
in the right column. Screen lines (in the 'extended' source) that actually contain modified code are marked with a vertical bar at
the beginning of the line.
Extending F83 resulted in 10words and
five source screens requiring modifications in the KERNEL68 file (see Listing
One), and 10 words and one source screen
in the EXTEND68 and CPU68000 files
(see Listing Two). No modifications were
required in either the METACOMPILER
or UTILITY files.
The effect on code size and execution
speed was as follows: a 640 byte growth in
size and a 3% increase in execution time
(based on the benchmark published in
Forth Dimensions VIIV4 - see Listing
Three).
To run this modified F83 (F83X). your
system must meet only three requirements.
First, the computer must be 680x0 based.
Second, the computer must use CP/M-68K
as its operating system. Thud, the Transient Program Area (TPA) of the CP/M68K system must define a region of memory that contains at least one 64K bounded
page of RAM (i.e., $F2 0000 through $F2
FFFF). (See Figure Four.)
For those of you interested in loading
F83X onto your system, I have written a
CPM-68K relocatable program called
GENF83X.REL that enables the user to
bootstrap a relocatable version of F83X,
called F83XREL.HEX, onto their system.
To install F83X, the users copies
GENF83X.REL and F83XREL.HEX onto
an empty, formatted diskette and places it
into the default drive. The user then exeForth Dimensions