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Relatively Simple CPU and 8085 microprocessor Instruction Set Architecture

Chapter 3. 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. 3.4. Relatively Simple Instruction Set Architecture. 3.5.

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Relatively Simple CPU and 8085 microprocessor Instruction Set Architecture

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  1. Chapter 3 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 3.4. • Relatively Simple Instruction Set Architecture 3.5. • 8085 Microprocessor Instruction Set Architecture • Analyzing the 8085 Instruction Set Architecture 3.6. • Summary

  3. 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

  4. InstructionSetArchitecture (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 theRelatively Simple CPU AC – accumulator register R – general purpose register /M[]– 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  • MVAC -- copies data in AC to register R • MOVR -- copies data from R to AC

  10. Data operation instruction for theRelatively Simple CPU AC – accumulator register R – general purpose register Z – zero flag

  11. Program control instruction for theRelatively Simple CPU Z – zero flag  -- 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 /M[]. These instructions each require 3 bytes in memory.

  13. Instruction formats for the Relatively Simple CPU byte 1 byte 2 byte 3 Example: 25: JUMP 1234 H instruction stored in memory: 25th byte25: 0000 0101 (JUMP) 26th byte26: 0011 0100 (34H) 27th byte27: 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 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

  16. 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

  17. InstructionSetArchitecture (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 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

  24. Data operation instruction for the 8085 microprocessor CY – carry flag

  25. Program control instruction for the 8085 microprocessor 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 byte 2 Two-byte Example: 25: MVI r, n instruction stored in memory: 25th byte25: 00xxx110 (MVI r) 26th byte26: xxxx xxxx (low-order memory) Specifies r

  28. byte 1 byte 2 byte 3 Three-byte Example: 25: LXI rp,  instruction stored in memory: 25th byte25: 00rp 0001 (LXI rp) 26th byte26: xxxx xxxx (low-order memory) 27th byte27: yyyy yyyy (high-order memory) Example: 25: LXI rp,  instruction stored in memory: 25th byte25: 00rp 0001 (LXI rp) 26th byte26: xxxx xxxx (low-order memory) 27th byte27: yyyy yyyy (high-order memory) Example: 25: MOV r1, r2 instruction stored in memory: 25th byte25: 0000 0001 (MOV) 26th byte26: xxxx xxxx (specifies r1) 27th byte27: yyyy yyyy (specifies r2) Specifies rp

  29. 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

  30. 3.5.4. 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. 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.

  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

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