The cassette version of FORTH

Since the cassette version has no use
of disc access routines, the FORTH words
in TABLE 1 have been removed.  These
words would otherwise occupy valuable
memory space, and their use would crash
the system.

TABLES 2 and 3 list words changed and
added to the system.  BLOCK and BUFFER
are identical.  BLOCK first looks for
a buffer identified by the block number
requested (?BUFF does this).  If none
of the buffers contains the requested
block, a "free" buffer should be assign
-ed.  ?ASSIGN looks for the first buff-
re not marked "updated", and assigns it
to the requested block number.  If all 
buffers are marked as updated, a message
will be printed.  See screens 1, 2 and 3
for BLOCK, ?BUFF and ?ASSIGN.

CINDEX lists the block (or screen)
number, and the first line, of the first
5 buffers.  If the first line contains
a non-printing character, a line of
blanks is printed.  If you have a 32K
RAM ABC80, you will have 21 more buffers
available.  Depress any key to list the
remaining buffers.  RETURN (key) will
exit CINDEX.

Each buffer of 768 bytes is preceeded by
a two byte header (which contains the 
screen number, with the most significant
bit set if the block has been "updated",
and the buffer is followed by a three
byte tail (two bytes in disc version).
These bytes are always set to zero,
which stops loading of a block.

ZTAIL puts zeroes in all the tail bytes.
ZHEAD puts zeroes in all the header
bytes.
EMPTY-BUFFERS is defined as ZHEAD ZTAIL.
The contents of the buffers is therefore
not destroyed by EMPTY-BUFFERS.


MEMORY ALLOCATION

In the disc version, FORTH begins at the
bottom of RAM (32768 or 49152) and 
occupies about 7.5K.  The disc buffers
(12 or 2) are located at the top of
memory (up to 62720) and the intervening
space is free for new FORTH words 
(+ terminal buffer and stacks).

In the cassette version, FORTH begins at
53082, occupying about 7K, with space 
above, up to 64256.  Between 49152 and
FORTH we find 5 buffers.  If an addi-
tional 16K of RAM is available, it is
 used for 21 buffers, from 32920 
to 49151.

COLD (the cold start routine) tests if
extra RAM is available (?START does 
this) and resets the value of FIRST
(lowest buffer address) and #BUFF.  All
tails are zeroed and the printer is set
to PR-OFF.  The remainder of COLD is
unchanged from the disc version.

EXIT FROM THE SYSTEM

The disc version of BYE is a system
reset (equivalent to pushing the "panic
button").  In the cassette version, BOFA
and EOFA are set to the limit between
the top buffer and FORTH.  Following BYE
you can run a BASIC program to read up-
dated buffers and write them out as a
text file, for future use.
$BAS has been added to FORTH.  BOFA and
EOFA are set to the current end of FORTH
(equal to HERE).  After using a short
BASIC program (to load additional
 screens, for example) a warm start
of FORTH will allow you to continue
using the new words loaded, or defined,
on the previous use of FORTH.


TABLE 1
FORTH words deleted from the cassette
version

ERR#      ?DISK      TRIAD     INDEX
FLUSH     R/W        SEC-WRITE SEC-READ
T&SCALC   SET-DRIVE  DR1       DR0
+BUF      DISK-ERROR DENSITY   SEC/BLK
TRACK     SEC        DRIVE


TABLE 2
FORTH words chnaged

BLOCK    BUFFER     BYE    EMPTY-BUFFERS
MESSAGE  COLD       LIMIT  VEDIT

TABLE 3
FORTH words added

CINDEX   ?ASSIGN    ?BUFF    ZWORD
ZHEAD    ZTAIL      ?START   $BAS


Best regards from

Robert Johnsen