CHAPTER 5

THE INSTRUCTION SET

5.1WHAT THE INSTRUCTION SET IS

A computer, no matter how sophisticated, can do only what it is instructed to do. A program is a sequence of instructions, each of which is recognized by the computer and causes it to perform an operation. Once a program is placed in memory space that is accessible to your CPU, you may run that same sequence of in- structions as often as you wish to solve the same problem or to do the same function. The set of instructions to which the 8085A CPU will respond is permanently fixed in the design of the chip.

Each computer instruction allows you to ini- tiate the performance of a specific operation. The 8085A implements a group of instructions that move data between registers, between a register and memory, and between a register and an 110 port. It also has arithmetic and logic instructions, conditional and unconditional branch instructions, and machine control in- structions. The CPU recognizes these instruc- tions only when they are coded in binary form.

5.2SYMBOLS AND ABBREVIATIONS:

The following symbols and abbreviations are used in the subsequent description of the

8085A instructions:

SYMBOLS

MEANING

accumulator

Register A

addr

16-bit address quantity

data

8-bit quantity

data 16

16-bit data quantity

byte 2

The second byte of the instruc-

 

tion

byte 3

The third byte of the instruc-

 

tion

port

8-bit address of an 110 device

r,r1,r2

One of the registers A,B,C,

 

D,E,H,L

DDD,SSS The bit pattern designating one of the registers A,B,C,D, E,H,L (DOD = destination, SSS = source):

DOD or REGISTER

SSSNAME

111A

000B

001C

010o

011E

100H

101L

rp

One of the register pairs:

B represents the B,C pair with

Bas the high-order register and C as the low-order register;

o represents the D,E pair with o as the high-order register and E as the low-order register;

H represents the H,L pair with

Has the high-order register and L as the low-order register;

SP represents the 16-bit stack pointer register.

RP

The

bit

pattern

designating

 

one of the register pairs

 

B,D,H,SP:

 

 

 

 

RP

 

REGISTER

 

 

 

PAIR

 

 

 

 

 

 

00

 

B-C

 

 

01

 

D-E

 

 

10

 

H-L

 

 

11

 

SP

rh

The first (high-order) ~egister

 

of a designated register pair.

rl

The

second

(low-order)

 

register

of a

designated

 

register pair.

 

* All mnemonics copyrighted © Intel Corporation 1976.

5-1

Page 86
Image 86
Intel MCS-80/85 manual H,L, H,Sp, 000 B, 011 E 100 H

MCS-80/85 specifications

The Intel MCS-80/85 family, introduced in the late 1970s, is a seminal collection of microprocessors that played a pivotal role in the early days of computing. The MCS-80 series, initially targeting embedded systems and control applications, gained remarkable attention due to its innovative architecture and flexible programming capabilities.

The MCS-80 family is anchored by the 8080 microprocessor, which was one of the first fully integrated 8-bit microprocessors. Released in 1974, the 8080 operated at clock speeds ranging from 2 MHz to 3 MHz and featured a 16-bit address bus capable of addressing up to 64KB of memory. The processor’s instruction set included around 78 instructions, providing extensive capabilities for data manipulation, logic operations, and branching.

Complementing the 8080 was a suite of support chips, forming the MCS-80 platform. The most notable among them was the 8155, which integrated a static RAM, I/O ports, and a timer, tailored for ease of designing systems around the 8080. Other support chips included the 8085, which provided improvements with an integrated clock generator, making it compatible with more modern designs and applications.

The MCS-85 series, on the other hand, revolves around the 8085 microprocessor, which provided a more advanced architecture. The 8085 operated at clock speeds of up to 6 MHz and came with a 16-bit address bus, similar to its predecessor. However, it introduced more sophisticated features, including an enhanced instruction set and support for interrupt-driven programming. These enhancements made the 8085 especially appealing to developers working in real-time processing environments.

The MCS-80/85 family utilized NMOS technology, known for its lower power consumption and higher performance compared to previous technologies like TTL. The family’s architecture allowed for easy interfacing with a variety of peripherals, making it a favorite for educational institutions and hobbyists embarking on computer engineering projects.

With its robustness, versatility, and affordability, the Intel MCS-80/85 microprocessors laid the groundwork for many subsequent microcomputer systems and applications. The legacy of this powerful family continues to influence modern microprocessor design, emphasizing the importance of reliable architecture in a rapidly evolving technology landscape.