Chapters 4 & 5:

LC-3 Computer

•Architecture

• Machine Instructions

• Assembly language

• Programming in Machine and Assembly Language

Overview

The LC-3 Computer

16 bit machine, word addressable, 64K or 65,536 locations

Computer Machine Instructions – Computer “native” instructions

- The basic instructions that all programs use on that computer

(The “atomic” unit of work done by a computer)

The Instruction Cycle

- The steps in the execution of a machine language instruction

(Fetch, Decode, Evaluate Address(es), Fetch operand(s), Execute,

and Store results)

LC-3 Data Path:

CombinationalLogic

State Machine

Storage

Data Path of the LC-3

LC-3 Memory Layout

x0000 – x2FFF System: Operating System programs, tables,

and data - Generally off limits to programmer

(Programs run in Supervisor mode)

x3000 – xFDFF User: User Program and Data Area

Area shared by users like you

(Nominally run in non-supervisor mode)

xFE00 – xFFFF Device: I/O Register Addresses

Pseudo memory used for input/output

R0-R7 Registers (16 bit)

LC-3 Memory Map

Computer Machine Instruction Formats

What is IN an instruction?• Operation code – what to do

• Input Operand(s) – where to get input operands (memory, registers)

• Output Operand(s) – Where to put results (memory, registers)

What are the major instruction types?• Data Movement (load, store, etc.)

• Operate (add, sub, mult,OR, AND, etc.)

• Control (branch, jump to subroutine, etc.)

The Instruction Cycle

Steps (or phases):• Fetch Next Instruction from Memory

(PC) (points to) next instruction

PC (PC) + 1

• Decode Fetched Instruction

• Evaluate Address (es) (find where the data is)

• Fetch Operand (s) (get data)

• Execute Operation

• Store Result (if specified)

The LC-3 Instruction Addressing Modes

• Register (Operand is in one of the 8 registers)

• PC-relative (Operand is “offset” from the (PC) )

• Base + Offset (Base relative) (Operand is “offset” from the contents of a register)

• Immediate (Operand is in the instruction)

• Indirect (The “Operand” actually points to the real address

– rather than being the operand)

The LC-3 Instruction Addressing Modes

• Register (Operand is in one of the 8 registers)

• Immediate (Operand is in the instruction)

• PC-relative (Operand is “offset” from the (PC) )

• Indirect (The “Operand” actually points to the real address – rather than being the operand)

• Base + Offset (Base relative) (Operand is “offset” from the contents of a register)

Note: no Direct Addressing defined in the LC-3

LC-3 Instructions (Fig 5.3 – Appendix a)

Operate Instructions

• Only three operations:

ADD, AND, NOT

• Source and Destination operands are:

Registers

ADD/AND (Register)

NOT (Register)

Note: Src and Dstcould be the same register.

ADD/AND (Immediate)

Note: Immediate field issign-extended.

Data Movement Instructions

•Load -- read data from memory to 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 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: immediate mode [1110 DR PCoffset9]

LD (PC-Relative)

ST (PC-Relative)

LDI (Indirect)

STI (Indirect)

LDR (Base+Offset)

STR (Base+Offset)

LEA (Immediate)

Branch Instruction

BR [0000 nzp PCoffset9]

• Branch specifies one or more condition codes• If the set bit is specified, the branch is taken:

– PC is set to the address specified in the instruction

– Target address is made by adding SEXT(IR[8:0]) to the PC

• If the branch is not taken: - the next sequential instruction (PC) is executed.

BR

/////////////

///// +

SEXT

Jump Instruction

JMP BaseR [1100 000 BaseR 000000]

• Jump is an unconditional branch -- always taken.

• Base– Address is contents of the register– Allows any target address.

TRAP

• Calls a service routine, identified by 8-bit “trap vector.”

• When routine is done, PC is set to the instruction following TRAP.

vector routine

x23 input a character from the keyboard

x21 output a character to the monitor

x25 halt the program

TRAPS

Using Branch Instructions

• Compute sum of 12 integers.Numbers start at location x3100. Program starts at location x3000.

R1 x3100R3 0R2 12

R2=0?

R4 M[R1]R3 R3+R4R1 R1+1R2 R2-1

NO

YES

R3: Accumulator for the sum of integers

R1: Array index pointer (Begin with location 3100)

R4: Temporary register to store next integer

R2: Loop counter (Count down from 12)

Sample Program

Address Instruction Comments

x3000 1 1 1 0 0 0 1 0 1 1 1 1 1 1 1 1 R1 x3100 (PC+0xFF)

x3001 0 1 0 1 0 1 1 0 1 1 1 0 0 0 0 0 R3 0

x3002 0 1 0 1 0 1 0 0 1 0 1 0 0 0 0 0 R2 0

x3003 0 0 0 1 0 1 0 0 1 0 1 0 1 1 0 0 R2 12

x3004 0 0 0 0 0 1 0 0 0 0 0 0 0 1 0 1 If Z, goto x300A (PC+5)

x3005 0 1 1 0 1 0 0 0 0 1 0 0 0 0 0 0 Load next value to R4

x3006 0 0 0 1 0 1 1 0 1 1 0 0 0 0 0 1 Add to R3

x3007 0 0 0 1 0 0 1 0 0 1 1 0 0 0 0 1 Increment R1 (pointer)

X3008 0 0 0 1 0 1 0 0 1 0 1 1 1 1 1 1 Decrement R2 (counter)

x3009 0 0 0 0 1 1 1 1 1 1 1 1 1 0 1 0 Goto x3004 (PC-6)

Example 1: Multiply

• This program multiplies two unsigned integers in R4 and R5.

x3200 0101010010100000x3201 0001010010000100x3202 0001101101111111x3203 0000011111111101x3204 1111000000100101

clear R2

add R4 to R2

decrement R5

R5 = 0?

HALT

No

Yes

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