RCA COSMAC User manual

Microprocessor
Products
User Manual for the
COSMAC Microprocessor

User Manual
for the COS MAC
Microprocessor
RCAISolid State DivisionISomerville, NJ 08876
Copyright 1975 by RCA Corporation
(All rights reserved under Pan-American Copyright Convention)
Printed in USA/5-75

2
Information furnished by RCA is believed to be accurate and
reliable. However, no responsibility is assumed by RCA for
its use; nor for any infringements of patents or other rights of
third parties which may result from its use. No license is
granted by implication or otherwise under any patent or
patent rights of RCA.
Trademark(s) Registered ®
M,arca(s) Registrada(s)

Table of Contents
Page No.
Introduction
General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Specific Features. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
System Organization
...................................................... 8
COSMAC Architecture and Notation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Instructions and Timing.
.. . . . . . . . .. . . . . . . . . . .. . . . . . . . . . • . . . . . . . . . . . . . . . . . . . . 12
Instruction Repertoire
Register Operations
...................................................... .
Memory Reference
....................................................... .
ALU Operations Using M(R(X))
ALU Operations Using M(R(P))
15
17
18
22
I nput/Output Byte Transfer ................................................ 24
Branching. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Control ................................................................ 27
Interrupt Handling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Instruction Utilization ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Memory and Control Interface
Memory Interface and Timing ...................................... . . . . . . . . . 33
Control Interfaces. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
I/O Interface
Programmed I/O
DMA Operation
Interrupt Control
Machine Code Programming
39
42
44
Sample System and Program. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
Useful Instructions
with X = P ............... ,.............................. 51
Interru pt Service . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
Branching Between Pages .................................................. 53
Subroutine Techniques 53
Common Program Bugs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
Appendixes:
A -
Instruction Summary .................................................. 58
B - State Sequencing ..................................................... 60
C - COSMAC Interface and ................................................ 61
Chip Connections
D - COSMAC
Timing Summary ............................................. 62
Index .................................................. '. . . . . . . . . . . . . . . . . . . . 63
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Foreword
The RCA Microprocessor (COSMAC) is an LSI CMOS 8-bit register-oriented central processing unit. It is
suitable for use in a wide range of stored-program computer systems and products. These systems may be
either special or general-purpose in nature.
This User Manual provides a detailed guide to the COSMAC Microprocessor. It is written for electrical
engineers,
and assumes no familiarity with computers. It describes the microprocessor architecture and its
set of simple, easy-to-use instructions. Examples are given to illustrate the operation of each instruction.
For systems designers, this manual illustrates practical methods of adding external memory and control
circuits. Because the processor is capable of supporting input/output (I/O) devices in polled, interrupt-
driven, and direct-memory-access modes, detailed examples are provided for the use of the I/O instructions
and the use of the I/O interface lines. The latter include direct-memory-access and interrupt inputs, external
flag inputs, command lines, processor state indicators, and external timing pulses.
This manual also describes
machine-code programming methods and gives detailed examples. Potential
programming errors are discussed, and various programming techniques are described, including interrupt
response, long branch, and subroutine linkage and nesting.
This basic
manual is intended to help design engineers understand the COSMAC Microprocessor and aid
them in developing simpler and more powerful products based on microprocessors. Users requiring infor-
mation on the operation of the RCA COSMAC Microprocessor software support system should refer to the
MPM-102 "Program Development Guide for the COSMAC Microprocessor".

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7
Introd uction
General
The COSMAC Microprocessor has been developed and tested within RCA in a wide variety of appli-
cations. COSMAC is suitable for use in business, education, entertainment, instrumentation, control,
communications, and other applications where stored program control is desired.
The RCA COSMAC Microprocessor is a CMOS byte-oriented central processing unit (CPU). It is suitable
for use in a wide range of stored-program computer systems or products. These systems can be either special
or general-purpose in nature. They are byte-oriented, a byte being eight bits.
COSMAC operations are specified by sequences of one-byte operation codes stored in a memory. These
operation codes are called instructions. Sequences of instructions, called programs, determine the specific
behavior
or function of a COSMAC-based system. System functions are easily changed by modifying the
program(s) stored in memory. This ability to change function without extensive hardware modification is
the basic advantage of a stored-program computer. Reduced cost results from using identical hardware
components (memory and microprocessor) in a variety of different systems or products.
The COSMAC microprocessor includes all of the circuits required for fetching, interpreting, and exe-
cuting instructions which have been stored in standard types of memories. Extensive input/output (I/O)
control features are also provided to facilitate system design.
Microprocessor
cost is only a small part of total system or product cost. Memory, input, output, power-
supply, system-control, and programming costs are also major considerations. A unique set of COSMAC
features combine to minimize the total system cost.
COSMAC's low-power, single-voltage CMOS circuitry minimizes power-supply and packaging costs.
High noise
immunity and wide temperature tolerance facilitate use in hostile environments.
COSMAC compatibility with standard, high-volume memories assures minimum memory cost and
maximum system flexibility for both current and future applications. Program storage requirements are
reduced by means
of an efficient one-byte instruction format.
The 40-pin COSMAC system interface is designed to minimize external I/O and memory control
circuitry. A single-phase clock, internal direct-memory-access (DMA) mode, flexible I/O instructions,
program interrupt, program load mode, and static circuitry are other COSMAC features explicitly aimed
at total system cost reduction. COSMAC does not require an external bootstrap ROM.
Microprocessor programming is facilitated by a variety of support programs or software. Extensive
support software and support hardware are available for use in developing COSMAC systems. Machine-
language programming
is sometimes indicated when only a few short programs need to be developed.
COSMAC provides a
set of efficient, easy-to-Iearn instructions which are simple to use.
The COSMAC microprocessor comprises
two conservatively designed LSI chips (one 40-pin and one
28-pin dual-in-line package). Appendix C shows the required interconnections for these two LSI chips and
summarizes the COSMAC system interface signals.

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User Manual for the
Specific Features
The advanced features and operating characteristics of the RCA COSMAC Microprocessor include:
• static COS/MOS circuitry, no minimum clock frequency
• full military temperature range
• high noise immun ity, wide operating-voltage range
• TTL compatibility
• 8-bit parallel organization with bidirectional data bus
• built-in program-load facility
• any combination of standard RAM/ROM via common interface
• direct memory addressing up to 65,536 bytes
• flexible programmed I/O mode
• program interrupt mode
• on-chip DMA facility
• four I/O flag inputs directly testable by branch instruction
• one-byte instruction format with two machine cycles for each instruction
• 59 easy-to-use instructions
• 16 x 16 matrix of registers for use as multiple program counters, data pointers, or data registers
System Organization
Fig. 1 illustrates a typical computer system incorporating the COSMAC microprocessor. Operations that
can be performed by COSMAC include:
a) control of input/output (I/O) devices,
b)
transfer of binary data between I/O and memory (M),
c) movement of data bytes between different memory locations,
d) interpretation or modification of bytes stored in memory.
N (41
I/O
____ jr
CPU
______ J
TIMING B MSC.
STATE CODE (21
DEPENDING ON
RAMIROM
SYSTEM
65536
TIMING (21
BYTES COS MAC CONTROL
MAX. MWRITE
DMA/INT. (31 CKTS
FLAGS (41
1/0
DEVICES
DATABUS(BI
92C5- 26554
Fig. 1 - Block diagram of typical computer system using the COSMAC microprocessor.

COSMAC Microprocessor _____________________________ _ 9
For example, COSMAC can control the entry of binary-coded decimal numbers from an input keyboard and
store them in predetermined memory locations. COSMAC can then perform specified arithmetic operations
using the stored numbers and transfer the results to an output display or printing device.
System input devices may include switches, paper-tape/card readers, magnetic-tape/disc devices, relays,
modems, analog-to-digital converters, photodetectors, and other computers. Output devices may include
lights, CRT /lED/liquid-crystal devices, digital-to-analog converters, modems, printers, and other computers.
Memory can comprise any combination of RAM and ROM up to a maximum of 65,536 bytes. ROM
(Read-Only Memory) is used for permanent storage of programs, tables, and other types of fixed data. RAM
(Random-Access Memory)
is required for general-purpose computer systems which require frequent program
changes.
RAM is also required for temporary storage of variable data. The type of memory and required
storage
capacity is determined by the specific application of the system.
Bytes are
transferred between ·1/0 devices, memory, and COSMAC by means of a common, bidirectional
eight-bit data bus.
Fifteen COSMAC I/O control signal lines are provided. Systems can use some or all of these signals
depending on required I/O sophistication. A four-bit N code is generated by the COSMAC input/output
instruction. It can be used to specify an I/O device to be involved in an I/O-memory byte transfer by means
of the data bus, or, alternatively, to specify whether an I/O byte represents data, an I/O device selection
code, an I/O status code, or an I/O control code. Use of the N code to directly specify an I/O device
permits simple, inexpensive
control of a small number of I/O devices or modes. Use of the N code to specify
the meaning of the word on the data bus facilitates systems incroporating a large number of I/O devices
or modes.
Four I/O flag inputs are provided. I/O devices can activate these inputs at any time to signal COSMAC
that a byte transfer is required, that an error condition has occurred, etc. These flags can also be used as
binary input lines if desired. They can be tested by COSMAC instructions to determine whether or not they
are active. Use of the flag inputs must be coordinated with programs that test them.
A program interrupt line can be activated at any time by I/O circuits to obtain an immediate COSMAC
response. The interrupt causes COSMAC to suspend its current program sequence and execute a prede-
termined sequence of operations designed to respond to the interrupt condtion. After servicing the inter-
rupt, COSMAC resumes execution of the interrupted program. COSMAC can be made to ignore the inter-
rupt line by resetting its interrupt-enable flip-flop (IE).
Two additional I/O lines are provided
for special types of byte transfer between memory and I/O devices.
These lines are called direct-memory-access (DMA) lines. Activating
the DMA-in line causes an input byte to
be immediately stored in a memory location without affecting the COSMAC program being executed. The
DMA-out line causes a byte to be immediately transferred from memory to the requesting output circuits. A
built-in memory pointer register is used to indicate the memory location for the DMA cycles. The program
sets this
pointer to an initial memory location. Each DMA byte transfer automatically increments the pointer
to the next higher memory location. Repeated activation of a DMA line can cause the transfer of any
number of consecutive bytes to and from memory independent of concurrent program execution.
I/O device circuits can cause data transfer by activating a flag line, the interrupt line, or a DMA line. A
program
must sample a flag line to determine when it becomes active. Activating the interrupt line causes an
immediate COSMAC response regardless
of the program currently in progress, suspending operation of that
program. Use of DMA provides the quickest response with least disturbance of the program.
A
two-bit COSMAC state code and two timing lines are provided for use by I/O device circuits. These
four signals permit synchronization of I/O circuits with internal COSMAC operating cycles. The state code
indicates whether COSMAC is responding to a DMA request, responding to an interrupt request, executing