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EECS 150 - Components and Design

Techniques for Digital Systems

Lec 21 – RTL Design Optimization

11/16/2004

David CullerElectrical Engineering and Computer Sciences

University of California, Berkeley

http://www.eecs.berkeley.edu/~cullerhttp://www-inst.eecs.berkeley.edu/~cs150

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Recall: Register Trans. Level Descriptions

• A standard high-level representation for describing systems.

• It follows from the fact that all synchronous digital system can be described as a set of state elements connected by combination logic (CL) blocks:

• RTL comprises a set of register transfers with optional operators as part of the transfer.

• Example:

regA regB

regC regA + regB

if (start==1) regA regC

• Personal style:– use “;” to separate transfers that

occur on separate cycles.

– Use “,” to separate transfers that occur on the same cycle.

• Example (2 cycles):

regA regB, regB 0;

regC regA;

reg regCL CL

clock input

output

option feedback

input output

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Components of the data path

• Storage– Flip-flops– Registers– Register Files– SRAM

• Arithmetic Units– Adders, subtraters, ALUs (built out of FAs or gates)– Comparators– Counters

• Interconnect– Wires– Busses– Tri-state Buffers– MUX

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Arithmetic Circuit Design (MT revisited)

• Full Adder

• Adder

• Relationship of positional notation and operations on it to arithmetic circuits

FA

A B Cin

Co S

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Recall: Computer Organization

• Computer design as an application of digital logic design procedures

• Computer = processing unit + memory system

• Processing unit = control + datapath

• Control = finite state machine– Inputs = machine instruction, datapath conditions

– Outputs = register transfer control signals, ALU operation codes

– Instruction interpretation = instruction fetch, decode, execute

• Datapath = functional units + registers + interconnect– Functional units = ALU, multipliers, dividers, etc.

– Registers = program counter, shifters, storage registers

– Interconenct = busses and wires

• Instruction Interpreter vs Fixed Function Device

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Datapath vs Control

• Datapath: Storage, FU, interconnect sufficient to perform the desired functions

– Inputs are Control Points– Outputs are signals

• Controller: State machine to orchestrate operation on the data path

– Based on desired function and signals

Datapath Controller

Control Points

signals

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Vector Op Example (more MT)

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List Processor Example

• RTL gives us a framework for making high-level optimizations.– Fixed function unit– Approach extends to instruction interpreters

• General design procedure outline:1. Problem, Constraints, and Component Library Spec.2. “Algorithm” Selection3. Micro-architecture Specification4. Analysis of Cost, Performance, Power5. Optimizations, Variations6. Detailed Design

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1. Problem Specification

• Design a circuit that forms the sum of all the 2's complements integers stored in a linked-list structure starting at memory address 0:

• All integers and pointers are 8-bit. The link-list is stored in a memory block with an 8-bit address port and 8-bit data port, as shown below. The pointer from the last element in the list is 0. At least one node in list.

I/Os:– START resets to head of list

and starts addition process.

– DONE signals completion

– R, Bus that holds the final result

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1. Other Specifications

• Design Constraints:– Usually the design specification puts a restriction on cost,

performance, power or all. We will leave this unspecified for now and return to it later.

• Component Library:component delay

simple logic gates 0.5nsn-bit register clk-to-Q=0.5ns

setup=0.5ns (data and LD)n-bit 2-1 multiplexor 1nsn-bit adder (2 log(n) + 2)nsmemory 10ns read (asynchronous read)zero compare 0.5 log(n)

(single ported memory)

Are these reasonable?

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2. Algorithm Specification

• In this case the memory only allows one access per cycle, so the algorithm is limited to sequential execution. If in another case more input data is available at once, then a more parallel solution may be possible.

• Assume datapath state registers NEXT and SUM.– NEXT holds a pointer to the node in memory.

– SUM holds the result of adding the node values to this point.

If (START==1) NEXT0, SUM0;

repeat {

SUMSUM + Memory[NEXT+1];

NEXTMemory[NEXT];

} until (NEXT==0);

RSUM, DONE1;

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A_SEL01

NEXT0

1

+

Memory

D

A

==0

+

01

SUM

NEXT_SEL

LD_NEXT

NEXT_ZERO

SUM_SEL

LD_SUM

0

1

0

3. Architecture #1Direct implementation of RTL description:

Datapath

Controller

If (START==1) NEXT0, SUM0; repeat { SUMSUM + Memory[NEXT+1]; NEXTMemory[NEXT];

} until (NEXT==0);RSUM, DONE1;

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4. Analysis of Cost, Performance, and Power

• Skip Power for now.

• Cost:– How do we measure it? # of transistors? # of gates? # of CLBs?

– Depends on implementation technology. Usually we are interested in comparing the relative cost of two competing implementations. (Save this for later)

• Performance:– 2 clock cycles per number added.

– What is the minimum clock period?

– The controller might be on the critical path. Therefore we need to know the implementation, and controller input and output delay.

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Possible Controller Implementation

START

COMPSUM

GETNEXT

DONE

LD_SUM

SUM_SEL

LD_NEXT

NEXT_SEL

DONE

A_SEL

START

START

START

NEXT_ZERO

• Based on this, what is the controller input and output delay?

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MT again…

• Longest path from any reg out to any reg input

3

2

2

4

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D Q

D Q

D Q

D Q

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4. Analysis of Performance

CLK-Q MUX

8-bit add memory 15-bit add

MUX

setup

.5 8 1 10 10 1 .5

31ns

CLK

NEXT

CLK

A_SEL

MUX

control output delay

memory

MUX

==0

control input delay

.5 101 1.51 1.5

15.5ns

COMPUTE_SUM state

GET_NEXT state

NEXT_ZERO

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Critical paths

A_SEL01

NEXT0

1

+

Memory

D

A

==0

+

01

SUM

NEXT_SEL

LD_NEXT

NEXT_ZERO

SUM_SEL

LD_SUM

0

1

0

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4. Analysis of Performance• Detailed timing:

clock period (T) = max (clock period for each state)

T > 31ns, F < 32 MHz

• Observation:COMPUTE_SUM state does most of the work. Most of the components

are inactive in GET_NEXT state.

GET_NEXT does: Memory access + …

COMPUTE_SUM does: 8-bit add, memory access, 15-bit add + …

• Conclusion:Move one of the adds to GET_NEXT.

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5. Optimization

• Add new register named NUMA, for address of number to add.

• Update RTL to reflect our change (note still 2 cycles per iteration):

If (START==1) NEXT0, SUM0, NUMA1;

repeat {

SUMSUM + Memory[NUMA];

NUMAMemory[NEXT] + 1,

NEXTMemory[NEXT] ;

} until (NEXT==0);

RSUM, DONE1;

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5. Optimization

• Architecture #2:

• Incremental cost: addition of another register and mux.

If (START==1) NEXT0, SUM0, NUMA1; repeat { SUMSUM + Memory[NUMA]; NUMAMemory[NEXT] + 1, NEXTMemory[NEXT] ; } until (NEXT==0); RSUM, DONE1;

A_SEL01

NEXT0

1

+

Memory

D

A

==0

+

01

SUM

NEXT_SEL

LD_NEXT

NEXT_ZERO

SUM_SEL

LD_SUM

0

1

0

01

NUMA

NEXT_SEL

LD_NEXT

1

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5. Optimization, Architecture #2• New timing:

Clock Period (T) = max (clock period for each state)

T > 23ns, F < 43Mhz

• Is this worth the extra cost?

• Can we lower the cost?

• Notice that the circuit now only performs one add on every cycle. Why not share the adder for both cycles?

CLK-Q

MUX

memory15-bit add

MUX

setup

.5 1 10 10 1 .5

23ns

CLK

NUMA

CLK

A_SEL

MUX

control output delay

memory

MUX

NUMA reg setup

.5 101 .51

21ns

COMPUTE_SUM state

GET_NEXT state

8-bit add

8

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5. Optimization, Architecture #3

• Incremental cost:– Addition of another mux and control. Removal of an 8-bit adder.

• Performance:– mux adds 1ns to cycle time. 24ns, 41.67MHz.

• Is the cost savings worth the performance degradation?

A_SEL01

NEXT0

1

Memory

D

A

==0

+

01

SUM

NEXT_SEL

LD_NEXT

NEXT_ZERO

SUM_SEL

LD_SUM

0

0

01

NUMA

NEXT_SEL

LD_NEXT

1

01ADD_SEL

1