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64 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ Operator Manual for the RCA CDS" CDP18S005
RAM Considerations
The eRA program occupies approximately 2.5
kilobytes of memory. It
is
supplied on UT20-
compatible paper tape for TTY use and on cassette
for use with the TI terminal.
eRA requires an additional
work
space
of at
least 1
I).ilobyte of RAM for
110
buflers
and,
particularly,
for the symbol
table which it constructs
and
references. Most of the available RAM work
space is
..
:used
for
the
symbol
table.
Each
entry has a variable
':
length
and contains the
symbol
(one
byte
per
:
character)
a
special
delimiter control byte, and a
-two-byte value. Becau e a
symbol i 1
to 6
bytes
in
length
, each
symbol
entry is 4
to
9
bytes
in
length.
eRA
makes
use
of additional
RAM
space
if it
has
been
provided,
by
starting
at
location
8000,
decrementing
down,
and
testing for RAM by
suc-
cessive
write/read operations. It
stops
when
the
first
RAM byte
is
found and asswnes that RAM exists
from
there down to location 0000. Therefore, any
added RAM must be contiguous to the original 4
kilobytes supplied.
In 1 kilobyte of work
space, .there
is room for
approximately 80 symbol table entries depending, of
course, on
the average
symbol
length. The required
work space is not a function of the number of
statements
in the
source
program being assembled.
Rather, it is a function of the number of
symbols
defined within it.
eRA is a
two-pass
assembler. That is normaUy it
reads the complete
source file
twice
to
complete an
assembly. During the fir
t
pass, the
symbol
table
is
constructed in
RAM
and printed on
the terminal.
Syntactic errors are flagged. On the
second
pass,
object code is generated
using
the symbol
table
values
just derived and an assembly
listing
is printed.
Addition program errors may be flagged on the
second pa
.
For example
the UNDEFINED
YMBOL
error
normally occurs
here.
Output Options
When the TI terminal is used, the output
or
object tape is generated automatically
during
the
second
pas . This terminal includes
means
by which
information printed on the printer may be different
from that
which
is
written on the output tape cassette.
In
tbis
case,
after
the second
pass, while the printed
listing contains all
the
information discussed earlier
(i.e., output
code values
and associated addresses
and
sequence
number and associated
source
lines),
the output tape
contains
only output code and addre s
information
in a format compatible
for subsequent
loading
via UT20. The output tape file
begins
with a
"1M'
message followed
by
a
sequence
olline which
have the UT20 semicolon loading format (where each
line begins with an address followed by a
sequence
of
hex digits to be loaded there, terminating with a
semicolon) .
If the
110
terminal is a teletypewriter, on the other
hand, then
when the
paper tape punch is activated,
information punched
on
the tape is the same as that
printed on the printer. Thus, if the punch were turned
on at the beginning of a
second
pass, the entire listing
would be written on the output paper tape. The
resulting tape
could
be used for subsequent loading if
desired because UT20 ignores any information on an
input !M line after the semicolon has occurred.
However, this procedure has
several
disadvantages.
The output tape is much longer than it has to he, with
most of the information on it extraneous. As a result,
the time to read it
(during subsequent
loading),
particularly at 10 characters per second, is normally
prohibitive. To handle this problem ,:\,hen
a
Teletype
terminal is
used,
eRA includes the ability to make
two kinds of
"second
passes" one of which omits the
printing of the
source
program lines and their
associated sequence numbers. Thus the normal
procedure to
generate
a paper tape
object
file is to
make a third pass (of the type just described) overthe
source file to
generate
an output tape which contains
the same information as would be written on a
magnetic tape cassette during
the
second pass.
Thus,
several
paper-tape,
second-pass
options exist
for the user. First, the normal second pass (generating
a listing) may be elected (option L) with the
'
p unch
off, waiting
for the third pass to generate the
output
tape (option H). Second, particularly for short
programs, the punch may be
activated
for this listing
pass - in which
case
the output tape will be longer
than necessary, but a third pass will not be required.
Third, particularly if an adequate listing
already
exists as the result of a previous run, the user may
elect the object-code-only option (H) on the second
pass,
with
the punch turned ON.
Prompt Messages
eRA begins any pass with a prompt message
which appears as
?F,H,L,U=
asking the user to type one of the letters shown to
define what eRA will do next. It is assumed that all
110
media involved - input and
output
magnetic or
paper tapes -
are
properly mounted. Typing F
selects
the first pass (symbol table construction). Typing L
selects
the Listing pass (the normal
second
pass).
Typing H selects the Hex-only listing pass (the
normal third pass
when
paper tape is used). Typing U
causes eRA to return control to UT20 (presumably
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