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CHAPTER 5 THE LC-3

CHAPTER 5 THE LC-3. Topics Memory organization Registers Instruction set Opcodes Data types Addressing modes Instructions Condition codes. LC-3: Memory. Memory Address size: 16 bits Address space: 2 16 = 65,536 locations #bits per location (addressability): 16 bits

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CHAPTER 5 THE LC-3

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  1. CHAPTER 5THE LC-3 Topics • Memory organization • Registers • Instruction set • Opcodes • Data types • Addressing modes • Instructions • Condition codes S. Barua – CPSC 240 sbarua@fullerton.edu http://sbarua.ecs.fullerton.edu

  2. LC-3: Memory Memory • Address size: 16 bits • Address space: 216 = 65,536 locations • #bits per location (addressability): 16 bits LC-3 operates on 16 bits of data. We refer to 16 bits as the word size in LC-3. S. Barua – CPSC 240 sbarua@fullerton.edu http://sbarua.ecs.fullerton.edu

  3. LC-3: Registers Registers • Temporary storage units • Eight general-purpose registers: R0 - R7 • Each register is 16 bits wide • Other registers • PC (Program Counter – 16 bits) • IR (Instruction Register – 16 bits) • Condition Codes How many bits are needed to uniquely identify a general-purpose register? S. Barua – CPSC 240 sbarua@fullerton.edu http://sbarua.ecs.fullerton.edu

  4. LC-3: Instruction Set Instruction usually contains two items: • Opcode & Operands Opcodes Specifies the operation the instruction requests the processor to execute. • Specified by 4 bits • Bits [15:12] of the instruction or IR [15:12] • 15 opcodes Operands Specifies data needed for the operation S. Barua – CPSC 240 sbarua@fullerton.edu http://sbarua.ecs.fullerton.edu

  5. LC-3: Instruction Set (Continued) Data Types • 16-bit 2’s complement integer Addressing Modes • Specifies how an operand can be accessed • 5 addressing modes in LC-3 • Immediate • Register • PC-relative • Indirect • Base+offset S. Barua – CPSC 240 sbarua@fullerton.edu http://sbarua.ecs.fullerton.edu

  6. Immediate & Register Addressing Mode Immediate addressing: Operand is included as part of the instruction Operand = SEXT (IR[4:0]) (SEXT means Sign extended to 16 bits) Register addressing: Operand is contained in a register S. Barua – CPSC 240 sbarua@fullerton.edu http://sbarua.ecs.fullerton.edu

  7. PC-Relative Addressing Mode PC-relative addressing: Instruction contains an offset value in IR[8:0] Memory address = PC* + SEXT (IR[8:0]) Operand = mem [ PC + SEXT(IR[8:0]) ] * This is the incremented PC value because PC is incremented as part of the FETCH phase S. Barua – CPSC 240 sbarua@fullerton.edu http://sbarua.ecs.fullerton.edu

  8. Indirect Addressing Mode Indirect addressing: Instruction contains an offset value in IR[8:0] Indirect address = PC* + SEXT (IR[8:0]) Memory address = mem [ PC + SEXT(IR[8:0]) ] Operand = mem [ mem [ PC + SEXT(IR[8:0]) ] ] * This is the incremented PC value because PC is incremented as part of the FETCH phase S. Barua – CPSC 240 sbarua@fullerton.edu http://sbarua.ecs.fullerton.edu

  9. Base+Offset Addressing Mode Base+offset addressing: Instruction contains a base register and an offset value in IR[5:0] Memory address = BaseR+ SEXT(IR[5:0]) Operand = mem [ BaseR+ SEXT(IR[5:0]) ] S. Barua – CPSC 240 sbarua@fullerton.edu http://sbarua.ecs.fullerton.edu

  10. LC-3: Instruction Set (Continued) Data Types • 16-bit 2’s complement integer Condition Codes LC-3 has 3 condition codes Some opcodes set/clear condition codes, based on result • N (Negative) = 1 if the result is negative • Z (Zero) = 1 if the result is zero • P (Positive) = 1 if the result is positive (> 0) S. Barua – CPSC 240 sbarua@fullerton.edu http://sbarua.ecs.fullerton.edu

  11. LC-3: Instruction Set (Continued) 3 different types of instructions in LC-3 • Operate instructions ADD, AND, NOT • Data movement instructions LD, LDI, LDR, LEA, ST, STR, STI • Control instructions BR, JSR/JSRR, JMP, RTI, TRAP S. Barua – CPSC 240 sbarua@fullerton.edu http://sbarua.ecs.fullerton.edu

  12. Operate Instructions Only three operations: ADD, AND, NOT • Source and destination operands are registers • These instructions do not reference memory • ADD and AND can use “immediate” addressing mode Immediate addressing: Operand is included as part of the instruction Register addressing: Operand is contained in a register S. Barua – CPSC 240 sbarua@fullerton.edu http://sbarua.ecs.fullerton.edu

  13. Data Movement Instructions • Load -- Read data from memory to register • Store-- Write data from register to memory • LD & ST – use PC-relative addressing mode • LDR & STR – use base+offset addressing mode • LDI & STI – use indirect addressing mode S. Barua – CPSC 240 sbarua@fullerton.edu http://sbarua.ecs.fullerton.edu

  14. Data Movement Instructions (Continued) Load Effective Address (LEA) Instruction contains an offset value in IR[8:0] Computes address using PC-relative addressing ( PC + SEXT (IR[8:0]) ) and stores the result into a register. Note: The address is stored in the register, not the contents of the memory location. S. Barua – CPSC 240 sbarua@fullerton.edu http://sbarua.ecs.fullerton.edu

  15. Control Instructions Control instructions alter the sequence of execution by changing the PC value. 3 types of control instructions in LC-3: • Conditional branch • Unconditional branch (jump) • TRAP S. Barua – CPSC 240 sbarua@fullerton.edu http://sbarua.ecs.fullerton.edu

  16. Control Instructions Conditional Branch Branch specifies one or more condition codes. If a specified condition is true PC ← PC + SEXT (IR [8:0]) ; branch to the new location else PC is not changed ; branch is not taken and the next sequential instruction is executed S. Barua – CPSC 240 sbarua@fullerton.edu http://sbarua.ecs.fullerton.edu

  17. Control Instructions (Continued) Unconditional Branch (or Jump) Always changes the PC TRAP Changes PC to the address of an OS “service routine” Routine will return control to the next instruction (after TRAP) S. Barua – CPSC 240 sbarua@fullerton.edu http://sbarua.ecs.fullerton.edu

  18. LC-3 Instruction Set The complete instruction set of LC-3 is explained in Appendix A (pages 521 – 545) of the CPSC 240 textbook. A few example instructions will be discussed in the class. Students are responsible for reviewing and understanding the complete instruction set of LC-3. S. Barua – CPSC 240 sbarua@fullerton.edu http://sbarua.ecs.fullerton.edu

  19. LC-3 Instruction Set - Example ADD R2, R3, R4 ;R2 ← R3 + R4 Where R2: Dst (destination register) IR[5] = 0 for Register R3: src1 (source register 1) addressing mode R4: src2 (source register 2) S. Barua – CPSC 240 sbarua@fullerton.edu http://sbarua.ecs.fullerton.edu

  20. LC-3 Instruction Set - Example ADD R2, R3, #7 ;R2 ← R3 + SEXT (00111) Where R2: Dst (destination register) IR[5] = 1 for Immediate R3: src1 (operand 1) addressing mode #7: Imm5 (imm. value as operand 2) S. Barua – CPSC 240 sbarua@fullerton.edu http://sbarua.ecs.fullerton.edu

  21. LC-3 Instruction Set - Example The condition codes specified by IR[11:9] are tested. For example, if IR[11] is set, then “n” is tested and if IR[11] is clear, “n” is not tested. BRz Loop ; branch to Loop if the last result was zero. If z = 1, PC ← PC + SEXT (IR [8:0]) This instruction employs PC-relative addressing mode. S. Barua – CPSC 240 sbarua@fullerton.edu http://sbarua.ecs.fullerton.edu

  22. LC-3 Instruction Set - Example This instruction employs Indirect addressing. STI R5, ADR ; mem [ mem [ PC + SEXT (IR [8:0]) ] ] ← R5 where R5: Src (Source register) S. Barua – CPSC 240 sbarua@fullerton.edu http://sbarua.ecs.fullerton.edu

  23. LC-3 Instruction Set - Example This instruction employs (Base + Offset) addressing. LDR R3, R6, #8 ; R3 ← mem [ R6 + SEXT (001000) ] where R3: Dst (Destination register) R6: Base (Base register) S. Barua – CPSC 240 sbarua@fullerton.edu http://sbarua.ecs.fullerton.edu

  24. LC-3 Instruction – Example Problems Convert the following LC-3 instruction into the corresponding machine codes: 1. AND R5, R1, R6 2. AND R5, R6, #14 3. BRp #9 4. LD R7, #8 5. JSR #12 6. LDR R4, R3, #12 7. LDI R4, #7 S. Barua – CPSC 240 sbarua@fullerton.edu http://sbarua.ecs.fullerton.edu

  25. Example Program on LC-3 Consider the following program that is loaded into memory starting at location x30FF. x30FF 1110 0010 0000 0001 x3100 0110 0100 0100 0010 x3101 1111 0000 0010 0101 x3102 0001 0100 0100 0001 x3103 0001 0100 1000 0010 • Give the equivalent assembly language code. • If the program is executed what is the value in R2 at the end of the execution? S. Barua – CPSC 240 sbarua@fullerton.edu http://sbarua.ecs.fullerton.edu

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