S. Barua – CPSC CHAPTER 5 THE LC-3 Topics Memory organization Registers Instruction set Opcodes Data types Addressing modes Instructions Condition codes

S. Barua – CPSC LC-3: Memory Memory Address size: 16 bits Address space: 2 16 = 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 LC-3 Instruction Set - Example This instruction employs (Base + Offset) addressing. LDR R3, R6, #8 ; R3 ← mem [ R6 + SEXT (001000) ] whereR3: Dst (Destination register) R6: Base (Base register)

S. Barua – CPSC 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 Example Program on LC-3 Consider the following program that is loaded into memory starting at location x30FF. x30FF x x x x (a) Give the equivalent assembly language code. (b) If the program is executed what is the value in R2 at the end of the execution?