Relatively Simple CPU and 8085 microprocessor Instruction Set Architecture

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Relatively Simple CPU and 8085 microprocessor
Instruction Set Architecture
Chapter 3

• Presented by Chi Yan Hung
• Class Cs 147 - sec 2 Fall 2001
• Prof Sin-Min Lee

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Topics to cover
3.4.

• Relatively Simple Instruction Set Architecture

3.5.

• 8085 Microprocessor Instruction Set Architecture
• Analyzing the 8085 Instruction Set Architecture

3.6.

• Summary

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3.4.
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
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Instruction Set Architecture (ISA) Memory
Model Registers Instruction set
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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
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• 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

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Instruction Set Data movement
instructions Data operation instructions Pro
gram control instructions
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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
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• 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

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• 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 0
AC accumulator register R general purpose
register Z zero flag
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• 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
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• 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.
  

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Instruction formats for the Relatively Simple CPU
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
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• 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
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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
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3.5.1-3
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
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Instruction Set Architecture (ISA) Memory
Model Registers Set Instruction Set
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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
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• 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.

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• 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 1s 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.

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

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Instruction Set Data movement
instructions Data operation instructions Pro
gram control instructions
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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 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
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Data operation instruction for the 8085
microprocessor
Instruction Operation Flags
ADD r A A r All
ADD M A A MHL All
INR r r r 1 Not CY
IN M MHL MHL 1 Not CY
DCR n r r - 1 Not CY
DCR M MHL MHL - 1 Not CY
XRA M A A ? MHL All
CMP r Compare A and r All
CMA A A? None
CY carry flag
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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 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
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• Note
• Each instruction is having an 8-bit
  instruction code.
• Some instructions have fields to
  specify registers, while others are fixed.

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Instruction formats for the Relatively Simple CPU
byte 1 byte 2
Two-byte
Example 25 MVI r, n instruction stored in
memory 25th byte 25 00xxx110 (MVI r) 26th
byte 26 xxxx xxxx (low-order memory)
Specifies r
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byte 1 byte 2 byte 3
Three-byte
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)
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
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• Example program using 8085 microprocessor
  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
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
i n
sum A ? A 0
sum sum i
i i - 1
IF i ? 0 THEN GOTO Loop
total sum
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3.5.4.
Analyzing the 8085 ISA The 8085 CPUs
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
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Advantages of the 8085s 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.
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The instruction set is fairly orthogonal.
E.g. no clear accumulator instruction Disadvan
tages of the 8085s ISA Like the Relatively
Simple CPU, it cannot easily process floating
point data.
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3.6.
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.
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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.
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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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THE END