Relatively Simple CPU and 8085 Relatively Simple CPU and 8085 microprocessor Instruction Set microprocessor Instruction Set

ArchitectureArchitecture

Presented by: Chi Yan Presented by: Chi Yan HungHung

Class: Cs 147 - sec 2 Fall Class: Cs 147 - sec 2 Fall 20012001

Prof: Sin-Min LeeProf: Sin-Min Lee

Topics to coverTopics to cover

• Relatively Simple Instruction Set ArchitectureRelatively Simple Instruction Set Architecture

• 8085 Microprocessor Instruction Set Architecture8085 Microprocessor Instruction Set Architecture

• Analyzing the 8085 Instruction Set ArchitectureAnalyzing the 8085 Instruction Set Architecture

• SummarySummary

Relatively Simple microprocessors, or CPU

Designed as an instructional aid and draws its features from several real microprocessors

Too limited to run anything as complex as personal computer

It has about the right level of complexity to control a microwave oven or other consumer appliance

Instruction Set Architecture (ISA)

Memory Model

Registers

Instruction set

Memory ModelThis microprocessor can access 64K ( = 216 ) bytes of memory

Each byte has 8 bits, therefore it can access 64K 8 bits of memory

64K of memory is the maximum limit, sometimes a system based on this CPU can have less memory

Use memory to map I/OSame instructions to use for accessing I/O devices and memory

Registers• Accumulator (AC), is an 8-bit general purpose

register

• Register R, is an 8-bit general purpose register. It supplies the second operand and also it can be use to store data that the AC will soon need to access.

• Flag Z, is an 1-bit zero flag. Z is set to 1 or 0 whenever an instruction is execute

• Other registers that cannot be directly accessed by programmer

Instruction SetData movement instructions

Data operation instructions

Program control instructions

• Data movement instruction for the Relatively Simple CPU

Instruction Operation

NOP No operation

LDAC AC = M[]

STAC M[] = AC

MVAC R = AC

MOVR AC = R

AC – accumulator register

R – general purpose register

/M[] – 16-bit memory address

• NOP -- performs no operation

• LDAC -- loads data from memory and stores it in the AC

• STAC -- copies data from AC to memory location

• MVAC -- copies data in AC to register R

• MOVR -- copies data from R to AC

• Data operation instruction for the Relatively Simple CPU

Instruction Operation

ADD AC = AC + R, If (AC + R = 0) Then Z = 1 Else Z = 0

SUB AC = AC - R, If (AC - R = 0) Then Z = 1 Else Z = 0

INAC AC = AC + 1, If (AC + 1 = 0) Then Z = 1 Else Z = 0

CLAC AC = 0, Z = 1

AND AC = AC R, If (AC R = 0) Then Z = 1 Else Z = 0

OR AC = AC R, If (AC R = 0) Then Z = 1 Else Z = 0

XOR AC = AC R, If (AC R = 0) Then Z = 1 Else Z = 0

NOT AC = AC, If (AC = 0) Then Z = 1 Else Z = 0AC – accumulator register R – general purpose register

Z – zero flag

• Program control instruction for the Relatively Simple CPU

Instruction Operation

JUMP GOTO JMPZ If (Z = 1) Then GOTO

JPNZ If (Z = 0) Then GOTO

Z – zero flag

-- 16-bit memory address

Note:

• Each instruction is having an 8-bit instruction code.

• LDAC, STAC, JUMP, JUMPZ, and JPNZ instructions all require a 16-bit memory address, represented by /M[]. These instructions each require 3 bytes in memory.

Instruction formats for the Relatively Instruction formats for the Relatively Simple CPUSimple CPU

Instruction codeLow-order 8 bits of

High-order 8 bits of

byte 1

byte 2

byte 3

Example:

25: JUMP 1234 H

instruction stored in memory:

25th byte 25: 0000 0101 (JUMP) 26th byte 26: 0011 0100 (34H) 27th byte 27: 0001 0010 (12H)

H -- in hexadecimal format

The Algorithm of the program

1: total = 0, i = 0

2: i = i + 1

3: total = total + i

4: IF i n THEN GOTO 2

What exactly this algorithm doing is: 1+ 2 + … + (n – 1) + n

• Example program using Relatively Simple CPU coding

The Relatively Simple CPU coding of the program

CLACSTAC totalSTAC i

Loop: LDAC iINACSTAC iMVACLDAC totalADDSTAC totalLDAC nSUBJPNZ Loop

total = 0, i = 0

i = i +1

total = total +1

IF i n THEN GOTO Loop

Relatively Simple microprocessors, or CPU

Designed as an instructional aid and draws its features from several real microprocessors

Too limited to run anything as complex as personal computer

It has about the right level of complexity to control a microwave oven or other consumer appliance

Instruction Set Architecture (ISA)

Memory Model

Registers Set

Instruction Set

Memory ModelThis microprocessor is a complete 8-bit parallel Central Processing Unit (CPU).

Each byte has 8 bits

Isolated I/O, input and output devices are treated as being separate from memory.

Different instructions access memory and I/O devices

Register Set• Accumulator A, is an 8-bit register.• Register B, C, D, E, H, and L, are six 8-bit

general purpose register. These registers can be accessed individually, or can be accessed in pairs.

• Pairs are not arbitrary; BC are a pair (16- bit), as are DE, and HL

• Register HL is used to point to a memory location.

• Stack pointer, SP, is an 16-bit register, which contains the address of the top of the stack.

• The sign flag, S, indicates the sign of a value calculated by an arithmetic or logical instruction.

• The zero flag, Z, is set to 1 if an arithmetic or logical operation produces a result of 0; otherwise set to 0.

• The parity flag, P, is set to 1 if the result of an arithmetic or logical operation has an even number of 1’s; otherwise it is set to 0.

• The carry flag, CY, is set when an arithmetic operation generates a carry out.

• The auxiliary carry flag, AC, very similar to CY, but it denotes a carry from the lower half of the result to the upper half.

• The interrupt mask, IM, used to enable and disable interrupts, and to check for pending interrupts

Instruction SetData movement instructions

Data operation instructions

Program control instructions

Data movement instruction for the 8085 microprocessor

Instruction Operation

MOV r1, r2 r1 = r2

LDA A = M[]

STA M[] = A

PUSH rp Stack = rp (rp SP)

PUSH PSW Stack = A, flag register

POP rp rp = Stack (rp SP)

POP PSW A, flag register = Stack

IN n A = input port n

OUT n Output port n =Ar, r1, r2 – any 8-bits register / M[] – memory locationrp – register pair BC, DE, HL, SP(Stack pointer)n – 8-bit address or data value

Data operation instruction for the 8085 microprocessor

Instruction Operation Flags

ADD r A = A + r All

ADD M A = A + M[HL] All

INR r r = r + 1 Not CY

IN M M[HL] = M[HL] + 1 Not CY

DCR n r = r - 1 Not CY

DCR M M[HL] = M[HL] - 1 Not CY

XRA M A = A M[HL] All

CMP r Compare A and r All

CMA A = A None

CY – carry flag

Program control instruction for the 8085 microprocessor

Instruction Operation

JUMP GOTO Jcond If condition is true then GOTO CALL Call subroutine at Ccond If condition is true then call subroutine at RET Return from subroutine

Rcond If condition is true then return from subroutine

cond – conditional instructionsNZ (Z = 0) Z (Z = 1) P (S = 0) N (S = 1)PO (P = 0) PE (P = 1) NC (CY = 0) C (CY = 1)

Z – zero flag, S – sign flag, P – parity flag, C – carry flag

Note:

• Each instruction is having an 8-bit instruction code.

• Some instructions have fields to specify registers, while others are fixed.

Instruction formats for the Relatively Instruction formats for the Relatively Simple CPUSimple CPU

Instruction codevalue

byte 1

byte 2

Example:

25: MVI r, n

instruction stored in memory:

25th byte 25: 00xxx110 (MVI r) 26th byte 26: xxxx xxxx (low-order memory)

Two-byte

Specifies r

Example:

25: MOV r1, r2

instruction stored in memory:

25th byte 25: 0000 0001 (MOV) 26th byte 26: xxxx xxxx (specifies r1) 27th byte 27: yyyy yyyy (specifies r2)

Instruction code

Low-order 8 bits

High-order 8 bits

byte 1

byte 2

byte 3

Three-byte

Example:

25: LXI rp,

instruction stored in memory:

25th byte 25: 00rp 0001 (LXI rp) 26th byte 26: xxxx xxxx (low-order memory) 27th byte 27: yyyy yyyy (high-order memory)

Example:

25: LXI rp,

instruction stored in memory:

25th byte 25: 00rp 0001 (LXI rp) 26th byte 26: xxxx xxxx (low-order memory) 27th byte 27: yyyy yyyy (high-order memory)

Specifies rp

The Algorithm of the program1: total = 0, i = 02: i = i + 13: total = total + i4: IF i n THEN GOTO 2

n + (n - 1) + … + 1

The 8085 coding of the program

LDA n

MOV B, A

XRA A

Loop: ADD B

DCR B

JNZ Loop

STA total

• Example program using 8085 microprocessor coding

i = n

sum = A A = 0

sum = sum + i

i = i - 1

IF i 0 THEN GOTO Loop

total = sum

Analyzing the 8085 ISA

The 8085 CPU’s instruction set is more complete than that of the Relatively Simple CPU. More suitable for consumer appliance.

Too limited to run anything as complex as personal computer

Advantages of the 8085’s ISA vs. Relative Simple CPU

It has the ability to use subroutines

It can incorporate interrupts, and it has everything the programmer needs in order to process interrupts.

The register set for the 8085 is mostly sufficient, thus less coding apply which will improve task completion.

The instruction set is fairly orthogonal. E.g. no clear accumulator instruction

Disadvantages of the 8085’s ISA

Like the Relatively Simple CPU, it cannot easily process floating point data.

Summary of ISA

The ISA specifies a. an instruction set that the CPU can processb. its user accessible registersc. how it interacts with memory

2. The ISA does not specify how the CPU is designed, but it specifies what it must be able to do.

3. The ISA is concerned only with the machine language of a microprocessor because CPU only executes machine language program, not any kind of high-level program.

When designing an ISA, an important goal is completeness:

a. instruction set should include the instructions needed to program all desired tasks.

b. instruction should be orthogonal, minimizing overlap, reducing the digital logic without reducing its capabilities within the CPU.

c. CPU should includes enough registers to minimize memory accesses, and improve performance.

5. An ISA should specifies the types of data the instruction set to process.

6. An ISA should specifies the addressing modes each instruction can use

7. An ISA should specifies the format for each instruction

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