Intel mcs-48 manual Summary, Architecture, Arithmetic Section

THE SINGLE COMPONENT MCS-48™SYSTEM

2.0 Summary

Sections 2.1 through 2.4 describe in detail the functional characteristics of the 8748 EPROM, 8048/8049 ROM and 8035/8039 single component microcomputers. Unless otherwise noted, details within these sec- tions apply to all .versions. Sections 2.5 through 2.11 describe the operation of the 8021, while Sections 2.12 through 2.21 de- scribe the 8022. This chapter is limited to those functions useful in single-chip imple- mentations of the MCS-48. Chapter 3 dis- cusses functions which allow expansion of program memory, data memory, and input- output capability.

Arithmetic Logic Unit

The ALU accepts 8-bit data words from one or two sources and generates an 8-bit result under control of the Instruction Decoder. The ALU can perform the following functions:

Add With or Without Carry

AND, OR, Exclusive OR

IncrementiDecrement

Bit Complement

Rotate Left, Right

Swap Nibbles

BCD Decimal Adjust

2.1 Architecture

The following sections break the 8048 into functional blocks and describe each in detail.

2.1.1 Arithmetic Section

The arithmetic section of the processor con- tains the basic data manipulation functions of the 8048 and can be divided into the following

blocks:

Arithmetic Logic Unit (ALU) Accumulator

Carry Flag Instruction Decoder

In a typical operation data stored in the accumulator is combined in the ALU with data from another source on the internal bus (such as a register or I/O port) and the result is stored in the accumulator or another register. The following is a more detailed description of the function of each block:

Instruction Decoder

The operation code (op code) portion of each program instruction is stored in the Instruction Decoder and converted to outputs which control the function of each of the blocks of the Arithmetic Section. These lines control the source of data and the destination register as well as the function performed in the ALU.

If the operation performed by the ALU results in a value represented by more than 8 bits (overflow of most significant bit) a Carry Flag is set in the Program Status Word.

Accumulator

The accumulator is the single most impor- tant data register in the processor, being one of the sources of input to the ALU and often the destination of the result of opera- tions performed in the ALU. Data to and from 1/0 ports and memory also normally passes through the accumulator.

2.1.2 Program Memory

Resident program memory consists of 1024 or 2048 words eight bits wide which are addressed by the program counter. In the 8748 this memory is user programmable and erasable EPROM; in the 8048/8049 the memory is ROM which is mask program- mable at the factory. The 8035/8039 has no internal program memory and is used with external devices. Program code is com- pletely interchangeable among the various versions. See Section 2.3 for EPROM pro- gramming techniques.

2-1