Memory Usage in Programs

Transcription

1 Memory Usage in Programs ECE2893 Lecture 4a ECE2893 Memory Usage in Programs Spring / 17

2 The Little Computer 3 (LC3) ECE2893 Memory Usage in Programs Spring / 17

3 The LC3 Instruction Set, Addition ADD DR, SR1, SR2 Add the contents of source registers SR1 and SR2, store the results in destination register DR. ADD R1, R2, R3 ADD DR, SR1, imm5 Add the contents of source registers SR1 and the 5 bit immediate value, store the results in destination register DR. The immediate value is specified in the low order 5 bits of the instruction. ADD R1, R2, #10 ECE2893 Memory Usage in Programs Spring / 17

4 The LC3 Instruction Set, Logical AND AND DR, SR1, SR2 Perform a bitwise logical AND of source registers SR1 and SR2, store the results in destination register DR. AND R1, R2, R3 AND DR, SR1, imm5 Perform a bitwise logical AND source registers SR1 and the 5 bit immediate value, store the results in destination register DR. The immediate value is specified in the low order 5 bits of the instruction. The immediate is sign extended. AND R1, R2, #7 ECE2893 Memory Usage in Programs Spring / 17

5 The LC3 Instruction Set, Branching Branch instructions conditionally cause the next instruction to be read from some address other than the next address. There are several variations, as follows: BRn Label Branch if negative BRz Label Branch if zero BRp Label Branch if positive BRp Label Branch if positive BR Label Branch unconditionally BRzp Label Branch if zero or positive BRnp Label Branch if negative or positive BRnz Label Branch if negative or zero BRnzp Label Branch if negative, zero, or positive The branch address is actually specified in the instruction as a nine bit offset from the current instruction address. This can be either negative or positive, allowing both backwards and forward branching. ECE2893 Memory Usage in Programs Spring / 17

6 The LC3 Instruction Set, Jumps Jump instructions unconditionally cause the next instruction to be read from some address other than the next address. Unlike the branch instructions, the address of the next instruction is not in a nine-bit offset, but rather found in one of the LC3 registers. There are two variations, but they are basically the same JMP BaseR Jump to the address found in the specified register. RET Jump to the adress found in register 7. Examples: JMP R4 (Jump to address specified in register 4) RET (Jump to address specified in register 7) ECE2893 Memory Usage in Programs Spring / 17

7 The LC3 Instruction Set, Jump to Subroutine Jump to subroutine instructions unconditionally cause the next instruction to be read from some address other than the next address. Further, the original value of the program counter is stored in register R7. There are two variations as follows. JSR Label Jump to the address specified by Label JSRR SR Jump to the address specified by the specified register. Examples: JSR MyLoop (Jump to address MyLoop) JSRR R4 (Jump to address specified in register 4) In both the above cases, original value of program counter is stored in register 7. ECE2893 Memory Usage in Programs Spring / 17

8 The LC3 Instruction Set, Load Register from Memory LD DR, Label Read the memeory address specified by Label and store the contents of that memory address in destination register DR LD R1, Length Reads the memory location Length and stores the value in R1. The address of the memory location is computed by adding the nine bit adddress (lower nine bits of the instruction) to the current program counter. This means the value to be read can be no more than bytes from the current memory location. ECE2893 Memory Usage in Programs Spring / 17

9 The LC3 Instruction Set, Load Register from Memory, Indirect LDI DR, Label Read the memeory address specified by Label (which is computed by adding the current Program Counter to the nine bit immediate value in the instructin). Then use the contents of that memory location as the address to read and store in destination register DR LDI R1, Length Reads the memory location Length, then reads the memory location specified by that location, and store the value in the specified register. Note that this instruction requires TWO memory accesses. ECE2893 Memory Usage in Programs Spring / 17

10 The LC3 Instruction Set, Load Register from Memory, Relative LDR DR, BASER, offset6 Read the memeory address specified by adding the contents of the specified base register and the six bit immeidate value. Store the results in the specified destinatino register. LDR R4, R2, #-5 Adds negative five to the contents of R2, then reads the memory at that location and stores the value in R4. ECE2893 Memory Usage in Programs Spring / 17

11 The LC3 Instruction Set, Load Effective Address LEA DR,Label Compute the address specified by adding the nine bit immediate value to the current program counter. Then store that address in the specified destination register (do not read memory). LEA R4,Length Computes the address of variable Length and stores that in register R4. ECE2893 Memory Usage in Programs Spring / 17

12 The LC3 Instruction Set, Logical NOT NOT DR SR Perform a bitwise logical NOT of source register SR and stores the results in destination register DR. NOT R5,R6 Perform a bitwise logical NOT of source register R6 and stores the result in destinatin registe R6. ECE2893 Memory Usage in Programs Spring / 17

13 The LC3 Instruction Set, Return from Subroutine RET Change the program counter unconditionally to the value found in register R7. ECE2893 Memory Usage in Programs Spring / 17

14 The LC3 Instruction Set, Store Register to Memory ST SR, Label Store the contents of the specified source register in the label specified. ST R2, SUM Store the contents of register R2 into memory location Sum. The address of the memory location is computed by adding the nine bit adddress (lower nine bits of the instruction) to the current program counter. This means the value to be stored can be no more than bytes from the current memory location. ECE2893 Memory Usage in Programs Spring / 17

15 The LC3 Instruction Set, Store Register to Memory, Indirect STI SR, Label Read the contents of the memory location specified by Label, then use that as the address to store the contents of source register SR. STI R2, NextAddress Read the contents of the memory location specified by NextAddress, then use that at the address to store the contents of register R2. ECE2893 Memory Usage in Programs Spring / 17

16 The LC3 Instruction Set, Store Register to Memory, Relative STR SR, BaseR, Offset Compute address by adding the contents of the specified base register (BaseR) to the specified six bit offset. Then store the contents of the specified source register (SR) in that address. STR R2, R3, #5 Compute an address by adding five to the contents of register R3, then stores the contents of register R2 in the computed address. then use that at the address to store the contents of register R2. ECE2893 Memory Usage in Programs Spring / 17

17 The LC3 Instruction Set, TRAP instructions The design of the LC3 has several ways to make operating system calls, primarily for keyboard and console input/output. These will be discussed in detail in class, and are covered in the pdf handout with details of the LC3 instruction set. ECE2893 Memory Usage in Programs Spring / 17