RCA COSMAC User manual

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LSI Systems Design
2nd Draft, February 1975
Copyright 1974, 1975 RCA ~orporation
Operator's
Manual
Solid State Technology Centel
Somerville, N. J

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LSI Systems Design
2nd Draft, February 1975
Copyright 1974, 1975 RCA Corporation
I
Operator's .
Manual
Solid State Technology Center
Somerville, N. J.

TABLE OF CONTEllTS
PAGE
INTRODUCTION
OPERATING AND PROGRAMMING THE MICROKIT
System Overview and Initial Operation...... 2
Instruction Set and Memory Organization........ 6
COSMAC Instruction Set................... 6
Memory
Organization...................... 6
Introduction to Utility Package - UT2.......... 8
Machine Language Programming .••••..•..•••••••.• 13
Programming Interface to CSDP ••••••••.••••••••• 18
UT2 Features and Calling Sequences •••.•••.••••• 26
UT2 Read and Type Routines ••••••.••••••.• 26
UT2 Register Storage Option .•••.•••••••.• 31
MICROKIT HARDWARE STRUCTURE AND INTERFACING
System Block Diagram •••••••••.••••....•••••••.• 32
Module Descriptions •....• · ••••.••••.•...••...••• 34
CPU Module............................... 34
Clock and Control Module ••.••••..•.•...•• 35
Address Latch Module •••••••••.•.•••.•••.. 37
5l2-Byte RAM Module ••..•••••..••.••••••.. 38
5l2-Byte PROM Module •••.••••.••••••.••••• 39
I/O Decoder Module ••••••••••••••••••• ~ ••• 40
Bus
Separator Module •••••••••••••••.••••• 41
Byte I/O Module •.•••••••••••.•••••••••••. 42
Terminal Module •.•••••••••••••••••••••••• 43
Card Nest and Signal-Naming Conventions .••••••• 44
Memory Module Addressing Details ••••••••••••••• 47
RCA ~1odules.............................. 47
Example A........................... 47
Example B........................... 48
Custom Memory Modules ••••••••••.••.•••••• 49
Input/Output Interfaces........................ Sl
Introduction............................. 51
The Module-Enable Philosophy and
Two-Level I/O ••••••••••••••••••••••• Sl
Interfacing Signals ••••••••••••.••••••••• 53
DMA. Input................................ S5
DMA. Output............................... S6
Interrupt. ............................... 56
Interfacing Examples..................... 57
Sample Program System •••••••••.•••.• 57
Bit Serial Interface -The Terminal
Module.......... .60
Interfacing Techniques and Precautions ••• 61
To Use External Clocks •••••••••••••• 61
External Flags EFI to EF4 ••••••••••• 61
Remote
Start/Stop •••••••••••••••••.. 62
Bus
Deseparator ••••••••••••••••••••• 62
Microkit Dynamic Characteristics •••. 62
Troubleshooting •• ,8 •••• ,.8.~., •. ~.~. 63
Hardware i Specifications ••••••••••••••• 1······· .. 64
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TABLE OF CONTErnS (cDtn'n)
PAGE
APPENDICES
A. TELETYPE HALF-TO-FULL DUPLEX CONVERSION
INSTRUCTIONS .••.• 66
B. UTZ ASSEMBLY LISTING •••••.•...•••••••.••.••..•.. 69
C. TTY -MODEM-MICROKIT SWITCHBOX .•••.••••••••••..••• 77
D. BACKPLANE WIRING SCHEDULE .•••.•••••••••••.•.•••. 79
E. MODULE LOGIC AND CIRCUIT DIAGRAMS ..•••••..••••.. 80-97
F. COSMAC INSTRUCTION SUMMARy .••••.••••••.••.•..... 98
G. ASCII - HEX TABLE •...•••.••...•.........•.....•. 99

TAB LEO F
FIGURE
NUMBER TITLE
FIG U RES
PAGE
1 I/O Typewriter Cable Connection 4
2
Elementary Hexadecimal Program Example 15
3 Source Code for Sample Program 22
4, Assembly Listing for Sample Program 23
5
Microkit Block Diagram 33
6
Block Diagram - CPU Module 34
7
Block Diagram - Clock and Control Module36
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
Block Diagram - Address Latch Module
Block Diagram - 5l2-Byte RAM Module
Block Diagram - 5l2-Byte PROM Module
Block Diagram - I/O Decoder Module
Block Diagram -Bus Separator Module
Block Diagram - Byte I/O Module
Block Diagram - Terminal Module
Custom 4K RAM Functional Diagram
DMA Input Example
DMA Output Example
Sample Program Interface
Sample Character Waveform
Model 33
Teletype Terminal Strip
Detailed Circuits for CPU - Terminal
Interface (Full Duplex Mode)
TTY-MODEM-KIT Switchbox
37
38
39
40
41
42
43
50
55
56
58
60
67
68
78

TAB LES
TABLE
NUMBER TITLE
PAGE
1
VT2
Register Initialization
29
2 VT2 Calling Sequences
30
3 Card Nest
Hodule Assingments
44
APPENDIX
D Backplane
Wiring Schedule 79
APPENDIX
F COSMAC Instruction
Summary 96
APPENDIX
GASCII -Hex
Table
97

I NT ROD UCT ION
The COS~~C Microkit and the COSMAC Software Development
Packages comprise an environment for the design of sys-
tems based ~n the COSMAC microprocessor. While the
Microkit may be viewed as a "stand-alone" machine, it
is specially structured to provide a testhed in which
hardware/software prototypes of systems containjng the
COSMAC microprocessor may he designed, built and tested.
The Microkit consists of a card nest with self-contained
power supply, an easy-to-use control panel and a basic
set of plug-in modules. It is packaged to promote easy
interfacing with external devices. These interfaces may
be custom-designed by the user, or for common peripheral
devices, they may be available from RCA as standard
optional modules.
The COSMAC Software Development Package, or CSDP, provides
a fundamental set of programming aids to facilitate the
coding and debugging of application programs. The. Microkit
includes means to transfer program material from the CSDP sys-
tem to the Microkit memory. There it can be executed by
the COS~~C microprocessor.
This manual is designed as a guide for the Microkit user.
It includes a detailed description of each of the avail-
able hardware modules as well as a complete explanation of
the functions available from utility programs, pre-written
into a programmable read-only memory (PROM), which is part
of the Microkit.
- I -

OPERATING AND PROGRAMMING THE MICROKIT
SYSTEM OVERVIEW AND INITIAL OPERATION
The nest for the Microkit consists of a set of printed
circuit board connectors with associated card guides
and a built-in power supply and control panel. The first
21 of the 34 available connector positions are assigned
to specific module* types. Many of their pins are pre-
wired using a printed circuit (PC) board backplane and a
small number of additional wire-wrapped connections.
(The latter were specifically made removable to permit
possible user modifications.) The last 13 of the con-
nectors are available for .any use.
Table 3 (p.44) contains a list defining to which module
each of the first 21 card positions is assigned. Eleven
of these positions are occupied with modules supplied in
the minimal Microkit, as indicated in the list. All
logic functions are implemented in COS/MOS. One card
contains the two-chip COSMAC microprocessor (the CPU --
"central processing unit"). Three memory cards are
included -- two 512-byte RAM's (random-access memories)
and one 512-byte PROM (programmable read-only memory)
containing the prewritten UTILITY PROGRAM, henceforth
called UT2. The crystal-controlled system clock and the
control-pinel interface reside on a card. Two cards are
used to provide appropriate data routing within the system.
An additional module is used for memory address latching.
Finally, three cards provide a fundamental I/O (input/out-
put) interface (including I/O device selection decoding,
a two-way byte-parallel I/O port and an I/O typewriter
serial interface). It should be emphasized that a spec-
ific, custom COSMAC controller need not require all of
these hardware facilities, if it is sufficiently simple.
Detailed descriptions of each of the available modules
appear elsewhere in this manual.
The supplied UT2 program includes a set of routines pro-
viding basic functions which virtually all users are ex-
pected to need. These include capabilities to load memory,
to read memory and to initiate program execution. Other
built-in services which are available from these routines
will be described later.
* We will use the terms "module", "card", or "board" inter-
changeably -- to represent a pluggable unit. It is in-
serted into the nest in a specified "slot" or "numbered
position". Each such slot has its own connector, mounted
in the nest.
- 2 -

SYSTEM OVERVIEW AND INITIAL OPERATION (cont'd)
The control panel provides an extremely simple user inter-
face. Depressing RESET initializes the CPU. Depressing
RUN UTILITY transfers control to UTZ. Depressing RUN PROGRAM
transfers control to memory location I (assuming some prev-
iously-loaded user program begins there). The RUN indicator
is illuminated whenever the CPU is running. The LOAD switch
is used to initiate byte-parallel memory loading using the
COSMAC "load mode" (discussed further later). This feature
is not utilized in the minimal Microkit, since program load-
ing and initiation are accomplished via standard services
supplied by UTZ.
The I/O typewriter interface supplied in the minimal Microkit
is designed to connect directly either to a user-supplied
ASR33 Teletype* or to a user-supplied Execuport** terminal
(or its electrical equivalent). The system is designed to
adjust to a variety of typewriter speeds. It will also
accommodate either FULL DUPLEX or HALF DUPLEX terminal oper-
ation. (For the reader's information, Appendix A provides
instructions for converting a Teletype from half duplex to
full duplex operation.)
Two I/O typewriter cables are supplied with the Microkit --
one for a Teletype, the other for an Execuport. They are
labeled appropriately. To connect the terminal, select the
proper cable and plug its receptacle labeled "PI" into the
appropriate connector mounted directly on the accessible end
of the Terminal board, in card position 20 (see Figure 1
below). The two connectors on the Terminal board are labeled
properly. The connector on the other end of the cable plugs
into the terminal. For a Teletype, remove the typewriter
cover and plug the cable into connector position 2(32) in
the array of white plastic Molex connectors located in the
back of the Teletype unit. For an Execuport, plug the Cinch
connector on the cable into the "External Device" receptacle***
(the bottom one) available on the back of the typewriter.
Figure 1, below, should clarify these connections. Note
that the plastic tie-downs, supplied in the Microkit, may be
used for anchoring the I/O typewriter cable.
NOTE: Due to specific hardware characteristics of the r/o typewriters
discussed above and of the terminal board, the Execuport Terminal should
be operated only in Half Duplex and the Teletype only in Full Duplex.
* Registered trademark, Teletype Corporation.
~* Registered trademark, Computer Transceiver Systems, Inc.
*** Models 310 and up only.
- 3 -

SYSTEM OVERVIEW AND INITIAL OPERATION (cont'd)
TERMINAL PLUG-IN
MODULE (~LOT .20)
MOUNTED 1/0
TYPEWRITER
CONNECTORS
WHITE PLASTIC
MOLEX
JACK
CABLE) ~
E +- rPil--L----__=___ P2
L..~ .LABELED ./
END TELETYPE- /
RECEPTACLE
END
COON(CT~
ACCESSIBLE
END
CINCH
CONNECTOR
Fig. 1: I/O Typewriter Cable Connection
(see text above)
WHITE PLASTIC
MOLEX JACK
INSIDE
TELETY~
IN POSITION -2
d (J21
-[t
'! EXTERNAL DEVICE·
CINCH RECEPTACLE
ON BACK OF
EXECUPORT
TERMINAL
Once the I/O typewriter has been properly connected
(with the Teletype operating in LINE mode, or the Execuport
in LOCAL mode) and Microkit power switched on (after the
supplied power cable has been installed), a simple test can
verify that the system is functioning properly. Depressing
RESET causes the RUN light to go off and the CPU to idle.
Following this by a depression of RUN UTILITY will cause
the RUN indicator to light. The system is now running,
with UT2 (the utility program) in control. This program
begins by reading the first keyboard input character to
define for itself the terminal character rate and whether
or not it should "echo" typed information to the typewriter
printer. If the terminal is operating in the FULL DUPLEX
mode, the user should begin by hitting the RETURN key on the
keyboard. For the HALF DUPLEX mode, he should hit the LINE
FEED key instead. The system will then respond with the
'*' prompt character. It now "knows" the essential charact-
eristics of the I/O typewriter. Reaching this simple stage
already verifies that most of the hardware is operating properly.
- 4 -

SYSTEM OVERVIEW AND INITIAL OPERATION (cont'd)
Even with little or no knowledge of the COSMAC command
repertoire, the user can further verify proper system
operation by loading, from the terminal, an elementary
test program. For example, a simple time-out loop can
be run, in which the RUN light goes off after a specific
elapsed time from the initiation of execution.
Each line of user keyboard input is terminated by a
depression of the RETURN key on the keyboard*. The test
time out program can be loaded into memory by typing in
!MI F8FFBl219191913A0400 (CR)
The system will reply with the prompt character. One can
verify proper loading by entering
?MltlA (CR)
The system will print the characters just entered (after
the memory location addressed - "0001", in this case) and
will return the prompt characxer again. The time out
program can then be run either by entering
$PI (CR)
or by depressing RESET followed by RUN PROGRAM. In either
case, the RUN indicator should go off after an elapsed
time of approximately 2.6 seconds. This establishes that
the read-write memory (RAM) is operational.
Having verified proper operation of the Microkit, you are
now ready to use the system. Some of the prerequisites
for this are discussed next.
* In various points in this manual, we will use (CR) or CR
at the end of an example of a user keyboard input to
denote the terminating carriage RETURN. Spaces in an
input line will be denoted by blanks in the example or,
for additional emphasis, by a tl symbol.
- 5 -

- 6 -
INSTRUCTION SET AND MEMORY ORGANIZATION
COS~~C INSTRUCTION SET
To write COSMAC programs, the user must first become
familiar with the internal logical structure and command
repertoire of the COS~~C microprocessor. It is, there-
fore, essential that the reader study first the inform-
ation provided in the COS~~C MICROPROCESSOR MANUAL before
going any further here. That document addltionally pro-
vides detailed information on the interface signals of
the COS~~C CPU, which is essential to an understanding of
the logical structure of the Microkit. It further provides
elementary interfacing examples which lead the reader
toward a comprehension of the internal interfaces within
the Microkit system.
It is also strongly recommended that the reader follow
the study of the COSMAC Microprocessor Manual with a
reading of the introductory section of the COS~~C SOFTWARE
~~NUAL, where hardware characteristics of the CPU are
summarized and reinforced.
We proceed, then, assuming the reader already has a detailed
familiarity with the COS~~C CPU. For completeness, the
Appendix includes a COSMAC INSTRUCTION SUMMARY and a table
relating ASCII characters to their hexadecimal representations.
MEMORY ORGANIZATION
To write elementary programs, the user must be aware of
which portions of the total memory address space in the
Microkit are available for his use. We cover only the basic
principles here. Further details are given later.
The total directly-addressable memory space (6S,S36 bytes)
may be considered as being divided into 16 blocks of 4096
bytes each, in which case we can divide the l6-bit memory
address, A[lS:O],* into two fields: A[lS:12] defining a
block number and A[ll:O] defining the byte within a block.
Further, a block may be considered either as consisting of
four banks of 1024 bytes each (in which case A[ll:lO] defines
a bank number within a block and A[9:0] is the address of a
byte within the selected bank) or as eight banks of S12 bytes
each (A[11:9] defining the bank number and A[8:0] identifying
the byte within the selected bank).
* We use the notation A[i:j] to denote the bit string
Ai,A(i-l), .• ~Aj. Thus, B[8:6] represents the three-bit
field B8,B7,B6.

MEMORY ORGANIZATION (cont'd)
Each memory plug-in module comprises a bank of memory.
When inserted in a given plug-in slot, external wiring
of appropriate pins on the connector in that position
defines its bank number (the range of addresses to which
it responds). An additional pin on the same connector
receives an overriding enable/disable signal. The module
responds only when it is enabled. These facilities allow
users to define abritary address ranges for the memory
modules they use. (Further wiring instructions are pre-
sented later.)
The standard Microkit is supplied with two S12-byte RAM
banks (using plug-in positions 11 and 12) enabled when
AlS=O
(the most significant address bit), and one Sl2-byte
PROM bank (position 4) enabled when AlS=l. The two RAM
banks are pre-wired to give a composite lK RAM whose bank
number is zero. The PROM in position 4 is also wired to
give it a bank number of zero. Thus, the RAM will respond
to addresses XOOO to X3FF, where X is 0 to 7 (most signif-
icant bit 0), while the PROM will respond to addresses
YOOO to YlFF, where Y is 8 to F. (most significant bit 1).
Normally, then, the RAM address range is 0000 to 03FF and
the PROM range is 8000 to 81FF (i.e., A[14:12] is normally
expected to be zero). Clearly, proper operation of programs
requires generation of memory addresses within those ranges
occupied by the physical memory modules present.
The supplied PROM bank contains the pre-written utility
program, UT2. Elementary aspects of its user interface are
described next.
- 7 -

INTRODUCTION TO UTILITY PACKAGE - UT2
The COS~~C Microkit is provided with a fixed PROM program,
which gives immediate access to all memory locations and
allows the user to begin program execution at any desired
point in memory. This program is known as UT2. Capability
is provided to load a program from an external source such
as the Teletype keyboard, a paper tape, or a timeshare
system. Tapes can be punched and used subse~uently to
reload memory. The various operations are executed through
the Microkit terminal, using three basic commands --
"?M" to read from memory and/or to punch a reloadable
paper tape, "!M" to modify memory contents or to load a
program into it, and "$P" to "proceed". with program execution.
In general, after the kit has been RESET, the user has two
choices: pushing RUN PROGRAM begins execution of his pro-
gram at location 0001, while pushing RUN UTILITY begins
execution of UT2 (at 8001). After depressing RUN UTILITY,
the user next types either a LF (line feed) or a CR (carriage
return), depending upon his installation. The latter is
used for FULL DUPLEX, the former for llALF DUPLEX. Besides
establishing the need to echo, UT2 uses this input to cal-
culate the timing parameters necessary to run the terminal,
thus allowing a single program to operate with wide vari-
ations in clock speed or terminal speed.
When UT2 is ready to accept acommand, it types out a prompt
character -- *.
To interrogate memory, the user types acommand such as
?M2FS 3
terminating with the "CR" -- carriage return. UT2 will
respond by printing out the contents of memory beginning at
location 02FS, printing three bytes out as two hex digits each.
Each line of output begins with the address and data is grouped
in 2-byte (4-digit) blocks. When necessary, new lines are
begun every 16 bytes, with the previous ending in semicolons.
The user may enter any number of digits to specify the
beginning location (leading zeroes are implied, if necessary).
If there are more than four, then only the last four are used.
The count of bytes to be typed out is in hex. Again, if more
than four digits are entered, only the last four are used.
This feature allows the user to correct a mistake. He simply
keeps typing, putting in the correct 4-digit values (230024=24).
When the user wants to punch a reloadable paper tape, he
requests amemory type-out, but before returning the carriage,
he turns on his tape punch and punches several null characters
as a header for the tape (using the REPEAT key and CONTROL-
SHIFT-P). UT2 will ignore the nulls. The resulting tape can
be used with the !M to load memory.
- 8 -

INTRODUCTION TO UTILITY PACKAGE - UT2 (cont'd)
In general, to enter data into memory, the user types
acommand such as
!M12F 434F534D4l43
which enters six bytes (two hex digits each) into memory
beginning at location l2F. This command is normally ter-
minated by a CR. Once again, the starting location is
determined by the last four digits entered. Data is en-
tered into memory after each two hex digits are typed. If
the user types an odd number of digits, then the last digit
is ignored, and an error message is typed out ('?').
This command provides two options to facilitate memory
loads. First, a string of data can be extended from line
to line by typing in a comma just before the normal CR.
(In this case the user must type his own CR-LF [carriage
return-line feed] to begin anew line.)
Example:
!M23 56789ABC,
DEF0123456,
3047
enters 11 successive bytes. Between successive hex pairs
while data is being entered, any non-hex character except
the comma (and semicolon, as will be discussed) is ignored.
This permits arbitrary LF's, spaces (for readability), nulls
(generated by the utility or by a time-share system to give
the carriage time to return), etc.
As a second optional form of data entry, a string of .input
data can be terminated by a semicolon (and aCR). The
utility program then expects more data to follow on the
next. line, but preceded by a new beginning address. The
line must have the format of an !M command, but with the
initial "!M" omitted. This option provides the mechanism
for reading in a paper tape previously punched out using
the 1M command. (Recall the format of multi-line ?M outputs
as discussed earlier.) The user types in "!M", positions
the paper tape in its header section, and hits TAPE READ.
(The utility ignores all non-hex characters following 1M,
which allows the CR, LF and nulls to be input from the
Teletype without disturbing the !M command.) Notice also
that the semicolon feature on input allows non-contiguous
memory areas to be loaded.
- 9 - ",

INTRODUCTION TO UTILITY PACKAGE - UTZ (cont'd)
The third utility command is $P. For example
$P6C
starts execution at location 6C with RO as the program
counter* (after the user hits CR and the utility program
provides a LF). The last-four-digits-in rule (pplies to
the address typed in.
In summary, the user has the following options after the
prompt character, *. He may type
?M[address]6[count]~
!M[non-hex] [address]6[data] [optional ,or;]~
(where "data" may have imbedded non-hex digits between
each hex pair) ~
or
$P[address]CR
UTZ ignores initial characters until it detects ?, !, or $.
Then, inputs which are not compatible with the above for-
mats will result in an error message.
*$P always begins with RO as program counter. This is
consistent with the fact that p=o after the COS~~C CPU
is RESET.
-10 -

INTRODUCTION TO UTILITY PACKAGE - UTZ (cont'd)
We present below a further detailed summary of in-
structions for use of UTZ, repeating the information
just presented in amore concise form.
1. After hitting "RUN UTILITY", hit either CR or LF:
a. For Half Duplex, hit LF.
b. For Full Dupl~x.!hit·CR.
(This sets up the bit-serial timing and specifies echo or not.)
Z. VTZ will return * as a prompt.
3. Following *, VTZ ignores all characters until one
of ?, $, or ! is typed in. Then:
4. Following? or ! must be M. Then VTZ waits for a hex
character. It then assembles an address (if more than
four hex digits are typed, only the last four are used).
Next, a space is required.
a. For?M addr 6 a hex count must follow (again, only
the last four digits are kept), and the command
is terminated by CR.
b. For!M addr 6 data must follow. An even number of
hex dIgIts IS required. Before each hex pair ar-
bitrary garbage is allowed, except for a CR, comma,
or semicolon. CR terminates the command, unless
immediately preceded by comma or generally preceded
by a semicolon.
i. In case of comma CR the user should insert
his own LF, and uTz continues to accept data.
This is a form of line continuation.
ii. In case of a semicolon, all following characters
are ignored until the CR is typed. Then the
user again should provide LF, and VTZ continues
as if he had begun anew command with !M --
i.e., optional garbage, then a starting address
must follow, then a space, and then data.
iii. The!M command can use as many continuation
lines as needed, mixed between the two types
if desired, and finally terminating with a CR
not preceded by comma or semicolon.
S. Following $ must be P, and then the starting address
(last four digits used if more than four typed in).
(If no address is entered, 0 is assumed.) Program
execution begins at this location using RO as program
counter.
- 11 -

INTRODUCTION TO UTILITY PACKAGE - UT2 (cont'd)
6. When a !M or ?M command is accepted and completed,
UT2 types another prompt chracter.
7. When UT2 sees bad syntax, it types back a? and
returns the carriage. If a mistake is maGe when
entering data (by typing an odd number of digits),
all data will have been entered except the last
hex digit. Note that the "only-last-four digits"
rule allows correction of an error without retyping
the whole command. (For example, a mistaken 234
can be corrected by continuing 2340235=0235.)
A bad command can be aborted by typing any illegal
character (except following !M or ?M or between
input hex digit data pairs -- in these cases type
any hex digit and then, for example, a period).
8. To punch reloadable paper tape, type ?M addr 6 count,
then a header of nulls (control-shift-P, 19nored
completely by UT2), then CR. When it's done, punch
some more nulls.
9. To load such a paper tape, type !M, position the
t~pe reader in the header, and turn on the tape reader.
- 12 -

MACHINE LANGUAGE PROGRAMMING
With an understanding of the details of the COS~~C CPU
and the material provided thus far, the reader is pre-
pared to begin using the Microkit in an elementary way.
For example, it is now possible to understand and pos-
sibly modify the time-out test program presented earlier
in this document. However, virtually any hexadecimal
(machine language) test program which the user may want
to write will require some I/O facilities (e.g., use of
the I/O typewriter). In this connection, we present
here some initial examples of how internal features of
UT2 can be utilized to reduce the user's programming
burden. Amore detailed description of the calling se-
quences of the various UT2 routines which may be of use
to the user are provided later.
To read a character from the I/O typewriter, the user
program should transfer control to READ* (in UT2), at
location 813E, making sure that R2 is pointing at a free
RAM location (say, 0000). After the typed character is
read, the UT2 routine will return by setting P to 5,
i.e., by executing the instruction D5 (making it most
convenient if the P of the calling routine were 5 to
begin with). The ASCII code for the input character
(with aa parity bit) will be in RF.I and also in D.
The memory location pointed at by R2 as well as registers
RE, RF, X and DF will have been changed in value (not
preserved over the call).
The READ routine normally uses R3 as its program counter,
so it is most convenient to branch to READ via a D3
instruction. R3.0 will also return with a modified value.
All other register contents will not be changed by the
call. Since the READ routine uses the values in registers
RC and RE which it has previously initialized, it is
essential that the user refrain from using these registers.
To cause a character to be typed onto the I/O typewriter,
the user program should transfer control, to TYPE5D (in UT2),
at location 8l9C, via a D3 instruction~ again making sure
that R2 is pointing at a free RAM location. As discussed
earlier, the calling P value should be 5 and, for this case,
the ASCII code for the output character should be an
immediate byte (i.e., the byte after the D3 call). After
typing the character, UT2 will advance R5 past the argument
byte and again return vi~ a D5 execution. M(R2), as well
as registers RE, RF, X, D, DF and R3.0 will return altered.
All other register values will be preserved. (For the
reasons mentioned earlier, the user should again refrain
from using registers RC and RE.)
*A list of key UT2 symbolic locations and their corresponding
absolute memory addresses appears in Table 2on p.30, where
UT2 calling sequences are discussed in greater detail.
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MACHINE LANGUAGE PROGRAMMING (cont'd)
The instructions given above are examples of UTZ
"calling sequences". Only the simplest information
sufficient to get the user started in some elementary
coding has been presented here. To intelligently use
all of the facilities available from UTZ, a programmer
should be familiar with the details provided n pp.Z6-3l
("UTZ Features and Calling Sequences"). In particular,
the TYPE linkage described above is not necessarily the
best to use. Other TYPE linkages which result in a
higher output rate and which use arguments passed via
registers are discussed in the section cited above.
Given the ability to execute simple typewriter I/O
functions, the user is now able to code elementary test
programs to further exercise the Microkit. As a simple
example, consider the routine described in the COS~~C
Microprocessor Manual (p.40) to read two bytes, compare
them and output the "larger" of the two. The routine
given there must be modified to exhibit register usage
compatible with the UTZ READ and TYPE calling sequences.
Further, the I/O instructions of that program must be
replaced with calls to the appropriate UTZ typewriter
interface routines. Figure Z, below, gives an example
of this program appropriately modified.
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