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Computer Architecture

Computer Architecture. Lecture by Engineer A. Lecturer Aymen Hasan AlAwady 24/2/2014 University of Kufa - Information Technology Research and Development Center. 1. 1. Instructions Execution. How the program in 8085 MPU is executed:

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Computer Architecture

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  1. Computer Architecture Lecture by Engineer A. Lecturer AymenHasanAlAwady 24/2/2014 University of Kufa - Information Technology Research and Development Center 1

  2. 1. Instructions Execution How the program in 8085 MPU is executed: • The userenters its instructionsin binary format into the memory. • The microprocessorthen reads these instructions and whatever data is needed from memory, executes the instructions and places the results either in memory or produces it on an output device. Assembly language

  3. 1.2 The three cycle instruction execution model To execute a program: • The microprocessor “reads” each instruction from memory, “interprets” it, then “executes” it. • To use the right names for the cycles: The microprocessor (1) fetcheseach instruction, (2) decodesit, then (3) executesit. • This sequence is continued until all instructions are performed. So How to write programs in 8085 MPU???

  4. 2. Machine Language Since the 8085 is an 8-bit device it can have up to 28(256) instructions. • However, the 8085 only uses 246 combinations that represent a total of 74 instructions. Most of the instructions have more than one format. These instructions can be grouped into five different groups: • Data Transfer Operations. • Arithmetic Operations. • Logic Operations. • Branch Operations. (Jump, call, Ret, etc…) • Machine Control Operations.

  5. 3. Instruction and Data Formats Each instruction has two parts. MVI A, 10H • The first part is the task or operation to be performed, his part is called the “opcode” (operation code MVI A). • The second part is the data to be operated on, called the “operand” 10H.

  6. 3.1 Data Transfer Operations These operations simply COPY the data from the source to the destination. • MOV, MVI, LDA, and STA They transfer: • Data between registers. EX: MOV A,B • Data Byte to a register or memory location. EX: MVI B,2H • Data between a memory location and a register. EX: MOV M,B • Data between an I\O Device and the accumulator. EX: IN 84H Note: The data in the source is not changed.

  7. 3.1 Data Transfer Operations. Cont... • STA (Store Accumulator Directly in Memory): This instruction is used to store the content of the accumulator to any address specified in the STA instruction. EX: STA 2000h In this instruction the content of A will be saved into address 2000h. • LDA (Load Accumulator Directly from Memory): this instruction is used to load the accumulator with content of the address specified with the instruction. EX: LDA 2020h The content of address 2020h will be copied to the accumulator.

  8. 3.1 LXI instruction • 8085 is dealing with 8-bit instructions. But 8085 provides an instruction to place the 16-bit data into the register pair in one step. (EX. BC). • LXI Rp, <16-bit address>(Load eXtendedImmediate). • For example the instruction LXI B,4020H will place the 16-bit number 4020 into the register pair B = 40H, C =20. • The upper two digits are placed in the 1stregister of the pair and the lower two digits in the 2nd.

  9. 3.1 The Memory “Register” Most of the instructions of the 8085 can use a memory location in place of a register. – The memory location will become the “memory” register M. • MOV M,B (copy the data from register B into a memory location) But Which memory location??? • The memory location is identified by the contents of the HL register pair. (EX. Lxi h,2000h (H=20h and L = 00h)) • The 16-bit contents of the HLregister pair are treated as a 16-bit address and used to identify the memory location.

  10. 3.1 Using the Other Register Pairs There is also an instruction for moving data from memory to the accumulator without disturbing the contents of the H and L register. • LDAX Rp(LoaD Accumulator eXtended) • STAX Rp(Store Accumulator eXtended) • LDAX: copy the 8-bit contents of the memory location identified by the Rp (BC or DE) register pair into the Accumulator. • STAX: copy the 8-bit content of accumulator to memory location identified by Rp (BC or DE) – This instruction only uses the BC or DE pair registers. – It does not accept the HL pair registers.

  11. 3.2 Arithmetic Operations Addition (ADD, ADI): with accumulator only • Any 8-bit number, Adi 01h (A=A+01h) • The contents of a register. ADD B (A=A+B) • The contents of a memory location. ADD M (A= A+M) • Can be added to the contents of the accumulator and the result is stored in the accumulator. Subtraction (SUB, SUI): with accumulator only • Any 8-bit number. Sui 04h (A= A-04h) • The contents of a register. Sub C (A=A-C) • The contents of a memory location. Sub M (A= A-M) • Can be subtracted from the contents of the accumulator. The result is stored in the accumulator.

  12. 3.2 Arithmetic Operations Related to Memory • These instructions perform an arithmetic operation using the contents of a memory location while they are still in memory. • ADD M (Add the contents of M to the Accumulator) • SUB M (Sub the contents of M from the Accumulator) • INR M and DCR M (Increment/decrement the contents of the memory location in place) • All of these use the contents of the HL register pair to identify the memory location being used.

  13. End of lecture 10

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