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Chapter 6

Chapter 6. Programming in Machine Language The LC-3 Simulator The LC-3 Editor. The LC-3 Computer a von Neumann machine. The Instruction Cycle:. Fetch: Next Instruction from Memory (PC)  (points to) next instruction PC  (PC) + 1

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Chapter 6

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  1. Chapter 6 • Programming in Machine Language • The LC-3 Simulator • The LC-3 Editor

  2. The LC-3 Computera von 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) • Load: 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 LC-3 CPU Registers: • 8 General Purpose Registers (R0 – R7) – Holds Data or Addresses • Program Counter (PC) - Points to the next instruction • Instruction Register (IR) – holds the instruction being executed • Program Status Word (PSW) – holds the status of the program being executed, including N Z P: Negative, Zero, Positive result of an operate instruction Memory Access Registers: • Memory Address Register (MAR) – Holds the address of a memory location being accessed • Memory Data Register (MDR) – Hold the data to be written into memory or the date read from memory Note: These are all 16 bit registers

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

  5. LC-3 Instructions • 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

  6. Operate Instructions • There are only three operate Instructions: -ADD Register mode [0001 DR SR1 0 00 SR2] Register/Immediate mode [0001 DR SR1 1 imm5] - AND Register mode [0101 DR SR1 0 00 SR2] Register/Immediate mode [0101 DR SR1 1 imm5] - NOT Register mode [1001 DR SR 111111] • The Source and Destination operands are: CPU Registers or Immediate Values

  7. 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] All have 2 or 3 operands

  8. Control Instructions • Go to New Location in Program – “GO TO” • BR: PC-relative mode [0000 NZP PCoffset9] • JMP: Indirect mode [1100 000 BaseR 000000] • Trap Service Routine Call • TRAP: Indirect [1111 0000 TrapVec8] • Jump toSubroutine (will be covered later) • JSR: PC-relative mode [0100 1 PCoffset11] • JSRR: Indirect mode [0100 000 BaseR 000000] • Return from Trap/Subroutine • RET: No operand [1100 000 111 000000] • Return from Interrupt(will be covered later) • RTI: No operand [1000 000000000000]

  9. TRAP Instruction • Calls a service routine, identified by 8-bit “trap vector.” • Register R7 is loaded with the incremented contents of the PC. • The PC is loaded with the address in the Trapvector Table at position “trapvector8” • R0 is typically used for passing values between the Program and the Trap Routine RET [1100 000 111 000000] • When service routine is done, an RET will load R7 (the incremented value of the PC before jumping to the TRAP routine) into the PC, and the program will continue with the next instruction after the TRAP, i.e. the program will “return” from the TRAP Routine. Note: an RET is a JMP Base-relative with Base = R7

  10. TRAPS See page 543.

  11. clear R2 add R4 to R2 decrement R5 R5 = 0? No Yes HALT Example:Program to multiply [R4] x [R5] and place the result in R2 R4 – Multiplicand R5 – Multiplier R2 – Accumulator

  12. LC-3 Instructions • 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

  13. clear R2 add R4 to R2 decrement R5 R5 = 0? No Yes HALT Example:Program to multiply [R4] x [R5] and place the result in R2 R2 <- 0 R2 <- R2 + R4 R5 <- R5 – 1 BRz x3201 HALT x3200 ?

  14. clear R2 add R4 to R2 decrement R5 R5 = 0? No Yes HALT Example:Program to multiply [R4] x [R5] and place the result in R2 R2 <- 0 R2 <- R2 + R4 R5 <- R5 – 1 BRz x3201 HALT x3200 0101010010100000 x3201 0001010010000100 x3202 0001101101111111 x3203 0000010111111101 x3204 1111000000100101

  15. clear R2 add R4 to R2 decrement R5 R5 = 0? No Yes HALT Example:Program to multiply [R4] x [R5] and place the result in R2 R2 <- 0 R2 <- R2 + R4 R5 <- R5 – 1 BRzp x3201 HALT x3200 0101010010100000 54A0 x3201 0001010010000100 1484 x3202 0001101101111111 1B7F x3203 0000010111111101 02FD x3204 1111000000100101 FO25

  16. LC3 Editor / Simulator Go to Author’s Web Site: http://www.mhhe.com/patt2 Get: • LC3 Edit • LC3 Simulator

  17. LC3 Edit Screen

  18. LC3 Edit Enter (or Load) the program into LC3 Edit - Store it as prog.bin for a binary file, or Store it as prog.hex for a hex file - Create a prog.obj file with the Editor

  19. LC-3 Simulator Screen

  20. LC-3 Simulator Open LC-3 Simulator - Load prog.obj - Load data.obj - Set breakpoint(s) or - Step through program

  21. LC-3 Simulator Open LC-3 Simulator - Load prog.obj - Load data.obj - Initialize values (PC, memory, registers) - Set breakpoint(s) - Step through program checking registers and “memory map” - Debug program

  22. clear R2 add R4 to R2 decrement R5 R5 = 0? No Yes HALT Example:Program to multiply [R4] x [R5] and place the result in R2 R2 <- 0 R2 <- R2 + R4 R5 <- R5 – 1 BRzp x3201 HALT x3200 0101010010100000 54A0 x3201 0001010010000100 1484 x3202 0001101101111111 1B7F x3203 0000011111111101 03FD x3204 1111000000100101 FO25

  23. Example:Program to multiply [R4] x [R5] and place the result in R2 Enter Program in Simulator and test it: • Enter Program • Run • Single Step • Add Breakpoints

  24. The Sum program in “binary” 0011000000000000 ;start x3000 x3000 1110001011111111 ;R1=x3100 x3001 0101011011100000 ;R3=0 x3002 0101010010100000 ;R2=0 x3003 0001010010101100 ;R2=R2+12 x3004 0000010000000101 ;If z goto x300A x3005 0110100001000000 ;Load next value into R4 x3006 0001011011000100 ;R3=R3+R4 x3007 0001001001100001 ;R1=R1+1 x3008 0001010010111111 ;R2=R2-1 x3009 0000111111111010 ;goto x3004 x300A 1111000000100101 ;halt

  25. The Sum program in “hex” 3000 ;start x3000 x3000 E2FF ;R1=x3100 x3001 56E0 ;R3=0 x3002 54A0 ;R2=0 x3003 14AC ;R2=R2+12 x3004 0405 ;If z goto x300A x3005 6840 ;Load next value into R4 x3006 16C4 ;R3=R3+R4 x3007 1261 ;R1=R1+1 x3008 14BF ;R2=R2-1 x3009 0FFA ;goto x3004 x300A F025 ;halt

  26. The Sum program Data in “hex” 3100 ; Begin data at x3100 x3100 0001 ; Loc x3100 x3101 0002 x3102 0004 x3103 0008 x3104 FFFF x3105 1C10 x3106 11B1 x3107 0019 x3108 0F07 x3109 0004 x310A 0A00 x310B 400F ; Loc x310B

  27. ExampleCompute the Sum of 12 Integers Program • Program begins at location x3000. • Integers begin at location x3100. R1  x3100R3  0 (Sum)R2  12(count) R2=0? R4  M[R1] R3  R3+R4R1  R1+1 R2  R2-1 NO YES R1: “Array” index pointer (Begin with location 3100) R3: Accumulator for the sum of integers R2: Loop counter (Count down from 12) R4: Temporary register to store next integer

  28. Example: Compute the Sum of 12 Integers Program Enter Program in Simulator and test it: • Enter Program • Enter Data • Run • Single Step • Add Breakpoints

  29. Example:# of Occurrences of an Inputted Char from a string R3 – ptr to char string, R1 – input char buffer, R2 – char count, R0 – output buffer

  30. Example:Counting Occurrences of a Character R3 – ptr to char string, R1 – input char buffer, R2 – char count, R0 – Keyboard char /Output display char

  31. HW 6: • After reading the Simulator manual: • Write a program to place the absolute value of the [R2] in R2. • The program should begin in memory location 3000 • Test it on the LC-3 Simulator. • Provide Simulator Snapshots with your homework. • 2) Write a program to read a value of N from memory and calculate N factorial. The result should be in R4. • The program should begin in memory location 3050 • N should be in memory location 3000 • Test the program on the LC-3 Simulator • Provide Simulator Snapshots with your homework.

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