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Reading – Papers Overview – An extreme of “SRAM” design is the register file. Register files are small SRAMs that are used heavily by the datapath. It serves as very local information that is fast to access. It often involves multiple ports for simultaneous access by a number of functional units/ALUs. – These design parameters lead to very different cell designs and performance targets. This set of notes reviews the basic concepts and shows an example of such a design.
Architecture – What is a register file – 2 basic approaches
What Is a Register File • • •
Fastest memory block available to the microprocessor. Stores intermediate results of the microprocessor units such as ALU & MMU Access speed is directly proportional to the performance of the processor.
Architecture: Multi-ported Design • At least 1 write port and 2 read ports – Accommodate a single ALU with 2-operand instructions. – r3 <= r2 + r1
• Superscalar designs – Multiple functional units access the register file.
• Enable different design constraints – Cell sizing – Different pre-charge of the read-port
Architecture: Multi-banking • Multi-porting has a large cost in peripheral circuits. – Replicate memory into many banks
• Homogenous – even division to a number of banks. – Faster access to each bank. – Smaller register size – More MUXing circuitry
Heterogeneous Multi-banking • Dividing the ports and registers unevenly to the banks. – Smaller bank for the critical data – Bigger bank for the noncritical data
• Prediction of critical data based on an algorithm similar to cache prediction. EE 215B
Design Example – Itanium register file
Itanium 2 Integer Register File
6 ALUs share 144 x 65 bit 22 ported general registers • 128 GRs + 16 Kernel Register aliased to R16-31 • 64 data path bits plus parity 12 read ports and 10 write ports – 8 active, 2 inactive • Active and inactive writes can occur simultaneously Datapath bypassing on write ports between multi-media (MMU) and integer execution units (IEU) IEU
1.37mm EE 215B
Integer RF Structure Address Driver
Decode Data Array Bitline Repeater
Parity State Machine EE 215B
Floating Point Register File
128 x 82 bit 18 ported general registers 8 Read Ports • 6 MAC data ports, 2 store data ports 10 write ports, 6 active 4 inactive • 2 MAC result ports , 4 load data ports
MAC 1.11mm EE 215B
Floating Point RF Structure
Bitline Repeater/Globa l Precharger
Data Array Parity State Machine
Decode EE 215B
Address Repeater Address Driver
Register File Timing
Write Write Bit Line Bitline Pre- Data Bypass discharge
Read READ Addr Decode
Write Addr Decode
Read Local Bitline Evaluate Read Global Bitline Evaluate
CK Phase 1 EE 215B
Read Local Precharge Read Global Precharge
CK Phase 2 FetzerISSCC05
Write Following Reads • •
Reading a register that is being written into occurs very often Itanium solution – Each register file access contains a READ followed by a WRITE. – No contention, the READ result can be used half-cycle early. Another common solution – Write bypass: • WRITE while READ results in a slow read since the cell is being flipped. • Bypass the READ with the WRITE information at the multiplexer.
Register File Decode highb highb
one read/write port self-timed pulse width control
address lowb matchb en
PCK2 sel[9:0] timer_enable
Wordline (en) is pulsed – PCK2X pulses each phase – Read followed by write WriteH is generated for the accessed register 16
Storage Cell WRITEH
writel thread ida
writel thread writel nb1 nb1
One storage node for each thread Storage node – Tristated by writel to assist NFET only pass gate writes. – writel drain connected PFETs provide extra pullup during a thread switch and make write easier.
writel thread Storage nodes
Register File READ/WRITE (1)
Buffered read – Isolate the cell from the read BL Additional buffering from write – Isolate stored data from read access. – Improve the write timing
wordline[9:0] EE 215B
Register File READ/WRITE (2)
Port sharing – Active thread READ shares wordlines with inactive WRITE – Reduce the number of total ports
wordline[9:0] read/write circuit EE 215B
Register File READ/WRITE (3) read bitline
Wordline conditioned by writel – Writel high, enables the read – Writel low, enables the pull up for the write.
wordline[9:0] read/write circuit EE 215B
Register File Organization • •
8 banks – 16 registers per bank 8 cells per bitline – 2 bitlines merge at the sense-amplifier – Small number of cells • Logic gate as the sense amplifiers • Pre-charged and evaluates low (high-skew)
200ps access time!
Register File Read Path PRECK
CK local0 read0 reg0
reg7 global bitline circuit
Pulldown in bitcell PRECK CK read EE 215B
READ Simulation •
Just over 200ps from CK to global bitline evaluate – PCK2X pulses twice per cycle – Matchb is the wordline enable signal. Local read/write signals generated from each wordline
Wordline Global BL Local BL EE 215B
WRITE Simulation To read port
and parity write
Writing a “1”
Writing a “0”
Floating Nodes During Write •The storage node in the inactive thread floats low during writes to the active thread. •At low frequency data could be lost so a timer is implemented on WRITEH to end the writes early TIMER CIRCUIT
writel writel nb0
writel RF Storage Node
•NCK rises and nr1 slowly drops. If the NCK phase is long enough enable drops low ending the write
Slow long L devices EE 215B
Switching Threads WRITEH
The READ/WRITE I/O ports look like large caps and there is a significant amount of charge sharing WRITEH is held at GND when thread/thread_b change values
FETs biti-1 shared with Read biti Buffering biti d0i d0i
biti EE 215B
Parity ripples through 32 stages in three clock cycles after a write (41 stages in four cycles in FPU) The two bit parity computation is 6.5 FETs per bit out of 109.5 (<6.0%) 28
Parity State Machine thread en
Thread Changed write
XOR computation tree b0
thread StoredParity ParityComp ParityError
The parity state machine is below the data array and gets the same inputs (wordlines/write/parity_in) as a bitcell Parity is continuously computed and checked – Register file outputs parity error. – Scan can observe a parity error before the register is read ParityError is read with a duplicate of a register read circuit 29
Register File Comparison Design
McKinley Integer ISSCC 2002
144 x 65bit
128 x 82bit
128 x 65bit
Parity SM Area
Summary • •
Register files are critical functional units similar to ALUs. – Determine the cycle-time of a processor Highly constrained memory design – Small number of entries – Large number of ports – Highly partitioned (tradeoff of #ports per cell versus many cells). Cell design is very unique. – Single-ended reads – Buffered reads – Multi-threading Sense-amplifiers are often digital logic gates Parity protection is increasingly critical for reliability. Reference 3