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Chapter 5 The LC-3

Chapter 5 The LC-3. Control Instructions. Used to alter the sequence of instructions (by changing the Program Counter) Conditional Branch branch is taken if a specified condition is true signed offset is added to PC to yield new PC else, the branch is not taken

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Chapter 5 The LC-3

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  1. Chapter 5The LC-3

  2. Control Instructions • Used to alter the sequence of instructions(by changing the Program Counter) • Conditional Branch • branch is taken if a specified condition is true • signed offset is added to PC to yield new PC • else, the branch is not taken • PC is not changed, points to the next sequential instruction • 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)

  3. Condition Codes • LC-3 has three condition code registers:N -- negativeZ -- zeroP -- positive (greater than zero) • Set by any instruction that writes a value to a register(ADD, AND, NOT, LD, LDR, LDI, LEA) • Exactly one will be set at all times • Based on the last instruction that altered a register

  4. Branch Instruction • Branch specifies one or more condition codes. • If the set bit is specified, the branch is taken. • PC-relative addressing:target address is made by adding signed offset (IR[8:0])to current PC. • Note: PC has already been incremented by FETCH stage. • Note: Target must be within 256 words of BR instruction. • If the branch is not taken,the next sequential instruction is executed.

  5. BR (PC-Relative) What happens if bits [11:9] are all zero? All one?

  6. Example from last class: Multiply value stored in R2 by 15 • ; initialize • 0101 000 000 1 00000 ; R0 <- R0 AND 0 • 0101 001 001 1 00000 ; R1 <- R1 AND 0 • ; code to repeat • 0001 001 001 000 010 ; R1 <- R1 + R2 • 0001 000 000 1 00001 ; R0 <- R0 + 1 • ; set condition codes • 0001 011 000 1 10001 ; R3 <- R0 – 15 • ; branch if R3 < 0 because count < 15 • 0000 100 111111100 ; PC<- PC- 4 if n==1 R0 <- R0 AND 0 R1 <- R1 AND 0 R1 <- R1 + R2 R0 <- R0 + 1 R3 <- R0 -15 true R3 < 0? n==1 false

  7. Refined Example: a better way to count • ; initialize • 0101 000 000 1 00000 ; R0 <- R0 AND 0 • 0101 001 001 1 00000 ; R1 <- R1 AND 0 • 0001 000 000 1 01111 ; R0 <- R0 + 15 • ; code to repeat • 0001 001 001 000 010 ; R1 <- R1 + R2 • 0001 000 000 1 11111 ; R0 <- R0 - 1 • ; set condition codes • 0001 011 000 1 10001 ; R3 <- R0 – 15 • ; branch if R0 >0 because count < 15 • 0000 001 111111101 ; PC<- PC- 3 if p==1 R0 <- R0 AND 0 R1 <- R1 AND 0 R0 <- R0 + 15 R1 <- R1 + R2 R0 <- R0 -1 true R0 > 0? p==1 false

  8. Iterative While Loop Do-While Loop true false

  9. Multiply Using the While Loop Structure R0 <- R0 AND 0 • ; initialize • 0101 000 000 1 00000 ; R0 <- R0 AND 0 • 0101 001 001 1 00000 ; R1 <- R1 AND 0 • 0001 000 000 1 01111 ; R0 <- R0 + 15 • ; branch if R0== 0 because count == 15 • 0000 010 000000011 ; PC<- PC+3 if z==1 • ; code to repeat • 0001 001 001 000 010 ; R1 <- R1 + R2 • 0001 000 000 1 11111 ; R0 <- R0 - 1 • ; branchunconditionally • 0000 111 111111100 ; PC<- PC-4 R1 <- R1 AND 0 R0 <- R0 + 15 true R0 == 0? z==1 false R1 <- R1 + R2 R0 <- R0 -1 always true true false

  10. Code for Iteration PC offset to address C Exact bits depend on condition being tested true false Unconditional branchto retest condition PC offset toaddress A Assumes all addresses are close enough that PC-relative branch can be used.

  11. Conditional If If-Else "hammock" "diamond"

  12. If conditional Problem statement: Increment R0 if R0 < R1 WRONG??? • ; form 2s complement of R0 • 1001 000 000 111111 ; R0 <- NOT R0 • 0001 000 000 1 00001 ; R0 <- R0 + 1 • ; R3 <- R1 + complement of R0 • 0001 011 000 000 001 ; R3 <- R0 + R1 • ; branch if R3 isneg or 0 • 0000 110 000000001 ; PC<- PC+1 if z==1 • or n==1 • ; increment R0 • 0001 000 000 1 00001 ; R0 <- R0 + 1 • 1111000000100101 ; halt R0 <- NOT R0 R0 <- R0 + 1 R3 <- R0 + R1 true R3 <= 0? z==1 n==1 false R0 <- R0 +1 false

  13. Analyze the Situation • Goal: Increment R0 if R0 < R1 • R0 and R1 can each be positive or negative • So  increment R0 if (R1-R0) is positive and branch to skip that step if (R1-R0) is zero or negative when we don’t want to increment -> we do want to branch

  14. If conditional (CORRECTED) Problem statement: Increment R0 if R0 < R1 • ; form 2s complement of R0 & store in R4 • 1001 100 000 111111 ; R4<- NOT R0 • 0001 100 100 1 00001 ; R4<- R4+ 1 • ; R3 <- R1 + complement of R0 (stored in R4) • 0001 011 100 000 001 ; R3 <- R4+ R1 • ; branch if R3 isneg or 0 • 0000 110 000000001 ; PC<- PC+1 if z==1 • or n==1 • ; increment R0 • 0001 000 000 1 00001 ; R0 <- R0 + 1 • 1111000000100101 ; halt R0 <- NOT R0 R0 <- R0 + 1 R3 <- R0 + R1 true R3 <= 0? z==1 n==1 false R0 <- R0 +1 false

  15. Code for Conditional PC offset toaddress C Exact bits depend on condition being tested Unconditional branchto Next Subtask PC offset toaddress D Assumes all addresses are close enough that PC-relative branch can be used.

  16. JMP (Register) • Jump is an unconditional branch -- always taken. • Target address is the contents of a register. • Allows any target address.

  17. TRAP • Calls a service routine, identified by 8-bit “trap vector.” • When routine is done, PC is set to the instruction following TRAP. • (We’ll talk about how this works later.) Warning: TRAP changes R7.

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