1. 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

2. Topics to cover
• Relatively Simple Instruction Set Architecture
• 8085 Microprocessor Instruction Set Architecture
• Analyzing the 8085 Instruction Set Architecture
• Summary

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

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

5. 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

6. 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

7. Instruction Set
• Data movement instructions
• Data operation instructions
• Program control instructions

8. • Data movement instruction for the
Relatively Simple CPU
Instruction Operation
NOP No operation
LDAC Γ AC = M[Γ]
STAC Γ M[Γ] = AC
MVAC R = AC
MOVR AC = R
AC – accumulator register
R – general purpose register
Γ/M[Γ] – 16-bit memory address

9. • NOP -- performs no operation
• LDAC -- loads data from memory and stores
it in the AC
• STAC -- copies data from AC to memory
location Γ
• MVAC -- copies data in AC to register R
• MOVR -- copies data from R to AC

10. • 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

11. • Program control instruction for the
Relatively Simple CPU
Instruction Operation
JUMP Γ GOTO Γ
JMPZ Γ If (Z = 1) Then GOTO Γ
JPNZ Γ If (Z = 0) Then GOTO Γ
Z – zero flag
Γ -- 16-bit memory address

12. 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 Γ/M[Γ]. These
instructions each require 3 bytes in
memory.

13. Instruction formats for the Relatively
Simple CPU
byte 1 Instruction code
byte 2 Low-order 8 bits of Γ
byte 3 High-order 8 bits of Γ
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

14. • 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

15. The Relatively Simple CPU coding of the program
CLAC
STAC total total = 0, i = 0
STAC i
Loop: LDAC i
INAC i = i +1
STAC i
MVAC
LDAC total
total = total +1
ADD
STAC total
LDAC n
SUB IF i ≠ n THEN GOTO Loop
JPNZ Loop

16. 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

17. Instruction Set Architecture (ISA)
• Memory Model
• Registers Set
• Instruction Set

18. 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

19. 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.

20. • 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.

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

22. Instruction Set
• Data movement instructions
• Data operation instructions
• Program control instructions

23. Data movement instruction for the 8085 microprocessor
Instruction Operation
MOV r1, r2 r1 = r2
LDA Γ A = M[Γ]
STA Γ M[Γ] = 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 Γ / M[Γ] – memory location
rp – register pair BC, DE, HL, SP(Stack pointer)
n – 8-bit address or data value

24. 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

25. Program control instruction for the
8085 microprocessor
Instruction Operation
JUMP Γ GOTO Γ
Jcond Γ If condition is true then GOTO Γ
CALL Γ Call subroutine at Γ
Ccond Γ If condition is true then call subroutine at Γ
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

26. Note:
• Each instruction is having an 8-bit instruction
code.
• Some instructions have fields to specify
registers, while others are fixed.

27. Instruction formats for the Relatively
Simple CPU
byte 1 Instruction code
Two-byte
byte 2 value
Example:
25: MVI r, n
Specifies r
instruction stored in memory:
25th byte 25: 00xxx110 (MVI r)
26th byte 26: xxxx xxxx (low-order memory)

28. byte 1 Instruction code
Three-byte byte 2 Low-order 8 bits
byte 3 High-order 8 bits
Example:
Example:
25:
25: LXI rp, Γr2
MOV r1,
Specifies rp
instruction stored in memory:
instruction stored in memory:
25th byte 25:
25th byte 25: 00rp 0001
0000 0001 (MOV)
(LXI rp)
26th byte 26:
26th byte 26: xxxx xxxx
xxxx xxxx (specifies r1)
(low-order memory)
27th byte 27:
27th byte 27: yyyy yyyy
yyyy yyyy (high-order r2)
(specifies
memory)

29. • 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 i=i-1
JNZ Loop IF i ≠ 0 THEN GOTO Loop
STA total total = sum

30. 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

31. 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.

32. • 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.

33. 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.

34. 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.

35. 6. An ISA should specifies the addressing modes each
instruction can use
7. An ISA should specifies the format for each instruction

36. THE END