Chapter 6

• Programming in Machine Language

• The LC-3 Simulator

• The LC-3 Editor

The LC-3 Computera von Neumann machine

Memory

PSW (Program Status Word): Bits: 15 10 9 8 2 1 0 | S| |Priority| | N| Z| P|

PSW

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)

The Instruction Cycle:

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

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

LC-3 InstructionsAddressing 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

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

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

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 to Subroutine (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]

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

vector Service routine (Partial List)

x23 input a character from the keyboard

x21 output a character to the monitor

x25 halt the program

TRAPS

See page 543.

Example:Program to multiply [R4] x [R5]

and place the result in R2

clear R2

add R4 to R2

decrement R5

R5 = 0?

HALT

No

Yes

R4 – Multiplicand

R5 – Multiplier

R2 – Accumulator

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

x3200 ?

clear R2

add R4 to R2

decrement R5

R5 = 0?

HALT

No

Yes

R2 <- 0R2 <- R2 + R4 R5 <- R5 – 1BRz x3201HALT

Example:Program to multiply [R4] x [R5]

and place the result in R2

x3200 0101010010100000x3201 0001010010000100x3202 0001101101111111x3203 0000010111111101x3204 1111000000100101

clear R2

add R4 to R2

decrement R5

R5 = 0?

HALT

No

Yes

R2 <- 0R2 <- R2 + R4 R5 <- R5 – 1BRz x3201HALT

Example:Program to multiply [R4] x [R5]

and place the result in R2

x3200 0101010010100000 54A0x3201 0001010010000100 1484x3202 0001101101111111 1B7Fx3203 0000010111111101 02FDx3204 1111000000100101 FO25

clear R2

add R4 to R2

decrement R5

R5 = 0?

HALT

No

Yes

R2 <- 0R2 <- R2 + R4 R5 <- R5 – 1BRzp x3201HALT

Example:Program to multiply [R4] x [R5]

and place the result in R2

LC3 Editor / Simulator

Go to Author’s Web Site:

http://www.mhhe.com/patt2

Get:• LC3 Edit• LC3 Simulator

LC3 Edit Screen

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

LC-3 Simulator Screen

LC-3 Simulator

Open LC-3 Simulator - Load prog.obj - Load data.obj

- Set breakpoint(s) or - Step through program

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

x3200 0101010010100000 54A0x3201 0001010010000100 1484x3202 0001101101111111 1B7Fx3203 0000011111111101 03FDx3204 1111000000100101 FO25

clear R2

add R4 to R2

decrement R5

R5 = 0?

HALT

No

Yes

R2 <- 0R2 <- R2 + R4 R5 <- R5 – 1BRzp x3201HALT

Example:Program to multiply [R4] x [R5]

and place the result in R2

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

The Sum program in “binary”

0011000000000000 ;start x3000x3000 1110001011111111 ;R1=x3100x3001 0101011011100000 ;R3=0x3002 0101010010100000 ;R2=0x3003 0001010010101100 ;R2=R2+12x3004 0000010000000101 ;If z goto x300Ax3005 0110100001000000 ;Load next value into R4x3006 0001011011000100 ;R3=R3+R4x3007 0001001001100001 ;R1=R1+1x3008 0001010010111111 ;R2=R2-1x3009 0000111111111010 ;goto x3004x300A 1111000000100101 ;halt

The Sum program in “hex”

3000 ;start x3000x3000 E2FF ;R1=x3100x3001 56E0 ;R3=0x3002 54A0 ;R2=0x3003 14AC ;R2=R2+12x3004 0405 ;If z goto x300Ax3005 6840 ;Load next value into R4x3006 16C4 ;R3=R3+R4x3007 1261 ;R1=R1+1x3008 14BF ;R2=R2-1x3009 0FFA ;goto x3004x300A F025 ;halt

The Sum program Data in “hex”

3100 ; Begin data at x3100x3100 0001 ; Loc x3100x3101 0002x3102 0004x3103 0008x3104 FFFFx3105 1C10x3106 11B1x3107 0019x3108 0F07x3109 0004x310A 0A00x310B 400F ; Loc x310B

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+1R2 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

Example: Compute the Sum of 12 Integers

Program

Enter Program in Simulator and test it:

• Enter Program

• Enter Data

• Run

• Single Step

• Add Breakpoints

Example:# of Occurrences of an Inputted Char from a string

Count = 0(R2 = 0)

Ptr = 1st file character(R3 = M[x3012])

Input charfrom keybd

(TRAP x23)

Done?(R1 ?= EOT)

Load char from file(R1 = M[R3])

Match?(R1 ?= R0)

Incr Count(R2 = R2 + 1)

Load next char from file(R3 = R3 + 1, R1 = M[R3])

Convert count toASCII character

(R0 = x30, R0 = R2 + R0)

Print count(TRAP x21)

HALT(TRAP x25)

NO

NO

YES

YES

R3 – ptr to char string, R1 – input char buffer, R2 – char count, R0 – output buffer

Example:Counting Occurrences of a Character

x300A 0 0 0 1 0 1 0 0 1 0 1 0 0 0 0 1 R2 R2 + 1

x300B 0 0 0 1 0 1 1 0 1 1 1 0 0 0 0 1 R3 R3 + 1

x300C 0 1 1 0 0 0 1 0 1 1 0 0 0 0 0 0 R1 M[R3]

x300D 0 0 0 0 1 1 1 0 0 0 0 0 0 1 0 0 Goto x3004

x300E 0 0 1 0 0 0 0 0 0 0 0 1 0 0 1 1 R0 M[x3013]

x300F 0 0 0 1 0 0 0 0 0 0 0 0 0 0 1 0 R0 R0 + R2

x3010 1 1 1 1 0 0 0 0 0 0 1 0 0 0 0 1 Print R0 (TRAP x21)

x3011 1 1 1 1 0 0 0 0 0 0 1 0 0 1 0 1 HALT (TRAP x25)

X3012 Starting Address of File

x3013 0 0 0 0 0 0 0 0 0 0 1 1 0 0 0 0 ASCII x30 (‘0’)

Address

Instruction Comments

x3000 0 1 0 1 0 1 0 0 1 0 1 0 0 0 0 0 R2 0 (counter)

x3001 0 0 1 0 0 1 1 0 0 0 0 1 0 0 0 0 R3 M[x3102] (ptr)

x3002 1 1 1 1 0 0 0 0 0 0 1 0 0 0 1 1Input to R0(TRAP x23)

x3003 0 1 1 0 0 0 1 0 1 1 0 0 0 0 0 0 R1 M[R3]

x3004 0 0 0 1 1 0 0 0 0 1 1 1 1 1 0 0 R4 R1 – 4 (EOT)

x3005 0 0 0 0 0 1 0 0 0 0 0 0 1 1 1 0 If Z, goto x300E

x3006 1 0 0 1 0 0 1 0 0 1 1 1 1 1 1 1 R1 NOT R1

x3007 0 0 0 1 0 0 1 0 0 1 1 0 0 0 0 1 R1 R1 + 1

X3008 0 0 0 1 0 0 1 0 0 1 0 0 0 0 0 0 R1 R1 + R0

x3009 0 0 0 0 1 0 1 0 0 0 0 0 1 0 1 1 If N or P, goto x300B

R3 – ptr to char string, R1 – input char buffer,

R2 – char count, R0 – Keyboard char /Output display char

HW 6:

After reading the Simulator manual:

1) 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.