Relatively Simple CPU and 8085
microprocessor Instruction Set
Architecture
Presented by: Chi Yan Hung
Class: Cs 147 - sec 2 Fall 2001
Prof: Sin-Min Lee
Topics to cover
• Relatively Simple Instruction Set Architecture
• 8085 Microprocessor Instruction Set Architecture
• Analyzing the 8085 Instruction Set Architecture
• Summary
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 Model

This 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/O
Same 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 Set

Data movement instructions

Data operation instructions

Program control instructions
•
Data movement instruction for the
Relatively Simple CPU
Instruction
Operation
NOP
No operation
LDAC 
STAC 
MVAC
MOVR
AC = M[]
M[] = AC
R = AC
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
SUB
INAC
CLAC
AC = AC + R, If (AC + R = 0) Then Z = 1 Else Z = 0
AC = AC - R, If (AC - R = 0) Then Z = 1 Else Z = 0
AC = AC + 1, If (AC + 1 = 0) Then Z = 1 Else Z = 0
AC = 0, Z = 1
AND
OR
XOR
AC = AC  R, If (AC  R = 0) Then Z = 1 Else Z = 0
AC = AC  R, If (AC  R = 0) Then Z = 1 Else Z = 0
AC = AC  R, If (AC  R = 0) Then Z = 1 Else Z = 0
NOT
AC = AC, If (AC = 0) Then Z = 1 Else Z = 0
AC – accumulator register
Z – zero flag
R – general purpose register
•
Program control instruction for the
Relatively Simple CPU
Instruction
Operation
JUMP 
JMPZ 
GOTO 
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
Simple CPU
byte 1
Instruction code
byte 2 Low-order 8 bits of 
byte 3 High-order 8 bits of 
Example:
25:
JUMP 1234 H
instruction stored in memory:
25th byte
25:
0000 0101
26th byte
26:
0011 0100
27th byte
27:
0001 0010
H -- in hexadecimal format
(JUMP)
(34H)
(12H)
•
Example program using Relatively Simple CPU coding
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
The Relatively Simple CPU coding of the program
CLAC
STAC total
STAC i
Loop: LDAC i
INAC
STAC i
MVAC
LDAC total
ADD
STAC total
LDAC n
SUB
JPNZ 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 Model

This 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 Set

Data movement instructions

Data operation instructions

Program control instructions
Data movement instruction for the 8085 microprocessor
Instruction
MOV r1, r2
Operation
r1 = r2
LDA 
STA 
PUSH rp
PUSH PSW
POP rp
POP PSW
IN n
OUT n
A = M[]
M[] = A
Stack = rp (rp  SP)
Stack = A, flag register
rp = Stack (rp  SP)
A, flag register = Stack
A = input port n
Output port n =A
r, r1, r2 – any 8-bits register
 / M[] – memory location
rp – 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
ADD M
INR r
IN M
DCR n
DCR M
XRA M
CMP r
CMA
A=A+r
A = A + M[HL]
r=r+1
M[HL] = M[HL] + 1
r=r-1
M[HL] = M[HL] - 1
All
All
Not CY
Not CY
Not CY
Not CY
All
All
None
CY – carry flag
A = A  M[HL]
Compare A and r
A = A
Program control instruction for the
8085 microprocessor
Instruction Operation
JUMP 
Jcond 
CALL 
GOTO 
If condition is true then GOTO 
Call subroutine at 
Ccond 
RET
Rcond
If condition is true then call subroutine at 
Return from subroutine
If condition is true then return from subroutine
cond – conditional instructions
NZ (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
Simple CPU
byte 1
byte 2
Two-byte
Instruction code
value
Example:
25:
MVI
r, n
instruction stored in memory:
25th byte
25:
00xxx110
26th byte
26:
xxxx xxxx
Specifies r
(MVI r)
(low-order memory)
byte 1
Three-byte byte 2
byte 3
Instruction code
Low-order 8 bits
High-order 8 bits
Example:
Example:
25:
25:
MOV rp,
r1,r2
LXI
instruction
instructionstored
storedininmemory:
memory:
25th
25thbyte
byte 25:
25: 00rp
00000001
0001
26th
26thbyte
byte 26:
26: xxxx
xxxxxxxx
xxxx
27th
27thbyte
byte 27:
27: yyyy
yyyyyyyy
yyyy
memory)
Specifies rp
(LXI
(MOV)
rp)
(low-order
(specifies memory)
r1)
(high-order
(specifies r2)
• Example program using 8085 microprocessor coding
The Algorithm of the program
1:
total = 0, i = 0
2:
i=i+1
n + (n - 1) + … + 1
3:
total = total + i
4:
IF i  n THEN GOTO 2
The 8085 coding of the program
LDA n
i=n
MOV B, A
XRA A
sum = A  A = 0
Loop: ADD B
sum = sum + i
DCR B
JNZ Loop
STA total
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
1.
The ISA specifies
a.
an instruction set that the CPU can process
b.
its user accessible registers
c.
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.
4.
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
THE END