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Chapters 5 - The LC-3

Chapters 5 - The LC-3. LC-3 Computer Architecture Memory Map Machine Instructions Address Modes Operate Instructions Data Move Instructions Programming in Machine Code. The LC-3 Computer a von Neumann machine. The Instruction Cycle:. Fetch: Next Instruction from Memory

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Chapters 5 - The LC-3

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  1. Chapters 5 - The LC-3 LC-3 Computer Architecture Memory Map Machine Instructions Address Modes Operate Instructions Data Move Instructions Programming in Machine Code

  2. The LC-3 Computeravon Neumannmachine The Instruction Cycle: • Fetch: Next Instruction from Memory • (PC)  (points to) next instruction • PC (PC) + 1 • Decode: Fetched Instruction • Evaluate: Instr &Address (es) • (find where the data is) • Fetch: Operand (s) • (get data as specified) • Execute: Operation • Store: Result • (if specified) PSW Memory PSW (Program Status Word): Bits: 15 10 9 8 2 1 0 | S| |Priority| | N| Z| P|

  3. Important Registers in the CPU • 8 General Purpose Registers – Holds Data or Addresses • PC – Points to the next instruction to be executed • IR – holds the instruction being executed • MAR – Holds the address of the memory location being accessed • MDR – Hold the data to be written into memory or the date read from memory • PSW (includes NZP) – holds the status of the program being executed (NZP: Negative, Zero, Positive result of an operate instruction)

  4. LC-3 Memory Map (64K of 16 bit words) 256 words (We will get to theses later) 256 words (We will get to these later) 23.5 K words ? 39.5 K words ? 512 words

  5. Computer Machine Instruction Formats • What is IN an instruction? • Operation code– what to do • Input Operand(s)– where to get input operands (memory, registers) • Output Operand(s)– Where to put results (memory, registers) • What are the major instruction types? • Data Movement (load, store, etc.) • Operate (add, sub, mult, OR, AND, etc.) • Control (branch, jump to subroutine, etc.)

  6. LC-3 Instructions (Fig 5.3 & Appendix a) • Addressing Modes • Register • (Operand is in one of the 8 registers) • PC-relative • (Operand is “offset” from where the PC points • - offsets are sign extended to 16 bits) • Base + Offset (Base relative) • (Operand is “offset” from the contents of a register) • Immediate • (Operand is in the instruction) • Indirect • (The “Operand” points to the real address of Operand • – rather than being the operand) • Note: The LC-3 has No Direct Addressing Mode

  7. Operate Instructions • There are only three operate Instructions: ADD, AND, NOT • The Source and Destination operands are: Registers

  8. ADD/AND (Register)

  9. NOT (Register) Note: Src and Dstcould be the same register.

  10. ADD/AND (Immediate) Note: Immediate field issign-extended to 16 bits.

  11. Data Movement Instructions • Load -read data from memory to a register • LD: PC-relative mode [0010 DR PCoffset9] • LDI: indirect mode [1010 DR PCoffset9] • LDR: base+offset mode [0110 DR BaseR offset6] • Store - write data from a register to memory • ST: PC-relative mode [0011 DR PCoffset9] • STI: indirect mode [1011 DR PCoffset9] • STR: base+offset mode [0111 DR BaseR offset6] • Load effective address – address saved in register • LEA: PC-relative mode [1110 DR PCoffset9]

  12. LD (LoaD PC-Relative Addressing)

  13. ST (STore PC-Relative Addressing)

  14. LDI (LoaD Indirect Indirect Addressing)

  15. STI (Store Indirect Indirect Addressing)

  16. LDR (LoaD Register Base+Offset Addressing)

  17. STR (Store Register Base+Offset Addressing)

  18. LEA (Load Effective Address PC-relative Addressing)

  19. Programming in Machine Code Write a program in machine code which: • Adds two integers • The program will be stored in the computer beginning in location x3000 • The integers, 26 and 67, will be stored in locations x3010 & x3011 • The result will be stored in location x3012

  20. Programming in Machine Code Modify the program to: • Subtract two integers • The program will be stored in the computer beginning in location x3000 • The integers, 26 and 67, will be stored in locations x3010 & x3011 • The result will be stored in location x3012

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