3-Software Design Basics in Embedded Systems

1. 3-Software Design Basics in Embedded Systems

2. Introduction • General-Purpose Processor • Processor designed for a variety of computation tasks • Low unit cost, in part because manufacturer spreads NRE over large numbers of units • Motorola sold half a billion 68HC05 microcontrollers in 1996 alone • Carefully designed since higher NRE is acceptable • Can yield good performance, size and power • Low NRE cost, short time-to-market/prototype, high flexibility • User just writes software; no processor design • a.k.a. “microprocessor” – “micro” used when they were implemented on one or a few chips rather than entire rooms

3. Processor Control unit Datapath ALU Controller Control /Status Registers PC IR I/O Memory Basic Architecture • Control unit and data-path • Note similarity to single-purpose processor • Key differences • Data-path is general • Control unit doesn’t store the algorithm – the algorithm is “programmed” into the memory

4. +1 Data-path Operations • Load • Read memory location into register Processor Control unit Datapath ALU • ALU operation • Input certain registers through ALU, store back in register Controller Control /Status Registers • Store • Write register to memory location 10 11 PC IR I/O ... Memory 10 11 ...

5. Processor Control unit Datapath ALU Controller Control /Status Registers PC IR R0 R1 I/O ... Memory 100 load R0, M[500] 500 10 101 inc R1, R0 501 ... 102 store M[501], R1 Control Unit • Control unit: configures the datapath operations • Sequence of desired operations (“instructions”) stored in memory – “program” • Instruction cycle – broken into several sub-operations, each one clock cycle, e.g.: • Fetch: Get next instruction into IR • Decode: Determine what the instruction means • Fetch operands: Move data from memory to datapath register • Execute: Move data through the ALU • Store results: Write data from register to memory

6. Control Unit Sub-Operations • Fetch • Get next instruction into IR • PC: program counter, always points to next instruction • IR: holds the fetched instruction Processor Control unit Datapath ALU Controller Control /Status Registers PC IR 100 R0 R1 load R0, M[500] I/O ... Memory 100 load R0, M[500] 500 10 101 inc R1, R0 501 ... 102 store M[501], R1

7. Control Unit Sub-Operations • Decode • Determine what the instruction means Processor Control unit Datapath ALU Controller Control /Status Registers PC IR 100 R0 R1 load R0, M[500] I/O ... Memory 100 load R0, M[500] 500 10 101 inc R1, R0 501 ... 102 store M[501], R1

8. Control Unit Sub-Operations • Fetch operands • Move data from memory to data-path register Processor Control unit Datapath ALU Controller Control /Status Registers 10 PC IR 100 R0 R1 load R0, M[500] I/O ... Memory 100 load R0, M[500] 500 10 101 inc R1, R0 501 ... 102 store M[501], R1

9. Control Unit Sub-Operations • Execute • Move data through the ALU • This particular instruction does nothing during this sub-operation Processor Control unit Datapath ALU Controller Control /Status Registers 10 PC IR 100 R0 R1 load R0, M[500] I/O ... Memory 100 load R0, M[500] 500 10 101 inc R1, R0 501 ... 102 store M[501], R1

10. Control Unit Sub-Operations • Store results • Write data from register to memory • This particular instruction does nothing during this sub-operation Processor Control unit Datapath ALU Controller Control /Status Registers 10 PC IR 100 R0 R1 load R0, M[500] I/O ... Memory 100 load R0, M[500] 500 10 101 inc R1, R0 501 ... 102 store M[501], R1

11. Processor Fetch ops Store results Control unit Datapath Fetch Decode Exec. ALU Controller Control /Status Registers 10 PC IR R0 R1 load R0, M[500] I/O ... Memory 100 load R0, M[500] 500 10 101 inc R1, R0 501 ... 102 store M[501], R1 Instruction Cycles PC=100 clk 100

12. Processor Control unit Datapath ALU Controller +1 Control /Status Registers Fetch ops Store results Fetch Decode Exec. 11 PC IR R0 R1 inc R1, R0 I/O ... Memory 100 load R0, M[500] 500 10 101 inc R1, R0 501 ... 102 store M[501], R1 Instruction Cycles PC=100 Fetch ops Store results Fetch Decode Exec. clk PC=101 clk 10 101

13. Processor Control unit Datapath ALU Controller Control /Status Registers PC IR R0 R1 store M[501], R1 Fetch ops Store results Fetch Decode Exec. I/O ... Memory 100 load R0, M[500] 500 10 101 inc R1, R0 501 11 ... 102 store M[501], R1 Instruction Cycles PC=100 Fetch ops Store results Fetch Decode Exec. clk PC=101 Fetch ops Store results Fetch Decode Exec. clk 10 11 102 PC=102 clk

14. Processor Control unit Datapath ALU Controller Control /Status Registers PC IR I/O Memory Architectural Considerations • N-bit processor • N-bit ALU, registers, buses, memory data interface • Embedded: 8-bit, 16-bit, 32-bit common • Desktop/servers: 32-bit, even 64 • PC size determines address space

15. Processor Control unit Datapath ALU Controller Control /Status Registers PC IR I/O Memory Architectural Considerations • Clock frequency • Inverse of clock period • Must be longer than longest register to register delay in entire processor • Memory access is often the longest

16. Pipelining: Increasing Instruction Throughput Wash 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 Non-pipelined Pipelined Dry 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 non-pipelined dish cleaning Time pipelined dish cleaning Time Fetch-instr. 1 2 3 4 5 6 7 8 Decode 1 2 3 4 5 6 7 8 Fetch ops. 1 2 3 4 5 6 7 8 Pipelined Execute 1 2 3 4 5 6 7 8 Instruction 1 Store res. 1 2 3 4 5 6 7 8 Time pipelined instruction execution

17. Superscalar Architectures • Performance can be improved by: • Faster clock (but there’s a limit) • Pipelining: slice up instruction into stages, overlap stages • Multiple ALUs to support more than one instruction stream • Superscalar • Scalar: non-vector operations • Fetches instructions in batches, executes as many as possible • May require extensive hardware to detect independent instructions (e.g. multiple ALUs) • VLIW (Very Long Instruction Word): each word in memory has multiple independent instructions • Relies on the compiler to detect and schedule instructions • Currently growing in popularity • CISC vs RISC?