Computer Science 210 Computer Organization

1. Computer Science 210Computer Organization A Data Path for von Neumann Processing

2. LC-3 Data Path Revisited Filled arrow = info to be processed Unfilled arrow = control signal

3. Data Path Components: Global Bus • Global bus • Carries 16-bit signal to many components • Inputs are tri-state devices, only signal when enabled • One signal is enabled at any time • Control unit decides which one • Any component can read from the bus • A register only captures data if write-enabled by the control unit

4. Data Path Components: Memory • Memory • Control and data registers for memory and I/O devices • MAR, MDR, control signal for read/write

5. LC-3 Data Path Revisited Filled arrow = info to be processed Unfilled arrow = control signal

6. Data Path Components: ALU • ALU • Accepts inputs from register file and sign-extended data from IR (immediate field) • Output goes to the bus • Output is used by condition code logic, register file, memory

7. LC-3 Data Path Revisited Filled arrow = info to be processed Unfilled arrow = control signal

8. Data Path Components: Register File • Register file • Two read addresses (Src1 and Src2) and one write address (Dst) • One input (actually a write) from the bus, the result of ALU operation or memory read • Two outputs (actually reads), used by ALU, PC, memory address • Data for store instructions passes through ALU

9. LC-3 Data Path Revisited Filled arrow = info to be processed Unfilled arrow = control signal

10. Data Path Components: PC and PCMUX • PC and PCMUX • Three inputs into PCMUX, output to PC • PC + 1 (fetch) • Address adder (BR, JMP) • Bus (TRAP)

11. LC-3 Data Path Revisited Filled arrow = info to be processed Unfilled arrow = control signal

12. Data Path Components: MAR and MARMUX • MAR and MARMUX • Two inputs into MARMUX, output to MAR • Address adder – LD/ST, LDR/STR • Zero-extended IR[7:0] – (TRAP)

13. LC-3 Data Path Revisited Filled arrow = info to be processed Unfilled arrow = control signal

14. Data Path Components: Logic • Condition code logic • Looks at value on the bus and generates N, Z, and P signals • Registers set only when control unit enables them, after ADD, AND, NOT, LD, LDI, LDR, LEA

15. LC-3 Data Path Revisited Filled arrow = info to be processed Unfilled arrow = control signal

16. Data Path Components: Control Unit • Control unit is an FSM • On each machine cycle, changes control signals for next phase of instruction processing • Who drives the bus? • Which registers are enabled? • Which operation should the ALU perform? • Logic decoder for the opcode, etc.