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Chapter 5
The LC-
LC-3 Computer
- Architecture
- Memory Map
- Instruction Set Architecture (ISA)
- Machine Instructions
- Address Modes
- Instructions - Operate - Data Move - Control
- Programming in Machine Code Docsity.com
The LC-3 Computer
a 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:
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LC-3 Memory Map
(64K of 1 6 bit words ) 256 words (We will get to theses today) 256 words (We will get to these later)
23.5 K words
39.5 K words
512 words
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LC-3 Instructions (Fig 5.3 & Appendix a)
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 isin the instruction)
(The “Operand”points to the real address of Operand
- rather than being the operand)
Note: The LC-3 has No Direct Addressing Mode Docsity.com
LC-3 Instructions (Fig 5.3 & Appendix a)
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 isin the instruction)
(The “Operand”points to the real address of Operand
- rather than being the operand)
Note: The LC-3 has No Direct Addressing Mode Docsity.com
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]
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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] Docsity.com
LC-3 Instructions (Fig 5.3 & Appendix a)
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 isin the instruction)
(The “Operand”points to the real address of Operand
- rather than being the operand)
Note: The LC-3 has No Direct Addressing Mode Docsity.com
BR
SEXT
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Jump Instruction
JMP BaseR [1100 000 BaseR 000000]
- Jump is an unconditional branch -- always taken.
- BaseR
- New PC contents (an Address) is the contents of the Base register
- Allows any target address!
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Compute the Sum of 12 Integers Program
- Program begins at location x3000.
- Integers begin at location x3100.
R1 ← x R3 ← 0 (Sum) R2 ← 12(count)
R2=0?
R4 ← M[R1]
R3 ← R3+R
R1 ← R1+
R2 ← R2-
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 Docsity.com
Sum integers from x3100 – x310B
Address Instruction Comments
x3000 1 1 1 0 0 0 1 0 1 1 1 1 1 1 1 1 R1 ← x31 00
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 ← 1 2
x3004 0 0 0 0 0 1 0 0 0 0 0 0 0 1 0 1 If Z, goto x300A
x3005 0 1 1 0 1 0 0 0 0 1 0 0 0 0 0 0 Load next value to R
x3006 0 0 0 1 0 1 1 0 1 1 0 0 0 1 0 0 Add to R
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 x
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
What happens at location 300A? Docsity.com
The Sum program in “binary”
x3000 1110001011111111 ;R1=x
x3001 0101011011100000 ;R3=
x3002 0101010010100000 ;R2=
x3003 0001010010101100 ;R2=R2+
x3004 0000010000000101 ;If z goto x300A
x3005 0110100001000000 ;Load next value into R
x3006 0001011011000100 ;R3=R3+R
x3007 0001001001100001 ;R1=R1+
x3008 0001010010111111 ;R2=R2-
x3009 0000111111111010 ;goto x
x300A 1111000000100101 ;halt
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The Sum program in “hex”
x3000 E2FF ;R1=x
x3001 56E0 ;R3=
x3002 54A0 ;R2=
x3003 14AC ;R2=R2+
x3004 0405 ;If z goto x300A
x3005 6840 ;Load next value into R
x3006 16C4 ;R3=R3+R
x3007 1261 ;R1=R1+
x3008 14BF ;R2=R2-
x3009 0FFA ;goto x
x300A F025 ;halt
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