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HPCA Glossary

Series of lecture notes on Computer Architecture from University of Texas.

15 stage pipeline: Additional Reference on Pipelines

1BP: 1-bit branch predictor

4 C's - compulsory Misses: the first time a block is accessed by the cache

4 C's - capacity Misses: blocks must be evicted due to the size of the cache. 

4 C's - coherence Miss: processors are accessing the same block. Processor A writes to the block. Even though Processor B has the block in its cache, it is a miss, because the block is no longer up-to-date.  Additional Reference on on Caches

4 C's - conflict Misses: associated with set associative and direct mapped caches - another data address needs the cache block and must replace the data currently in the cache.

advanced cache optimizations: Additional Reference on Advanced Cache Optimizations

ALAT: advance load table - stores advance information about load operations Additional Reference on ALAT

aliasing: in the BTB, when two addresses overlap with the same BTB entry, this is called aliasing. Aliasing should be kept to <1%. 

ALU: arithmetic logic unit

AMAT: average memory access time

AMAT: Average Memory Access Time = hit time + miss rate * miss penalty

Amdahl's Law: an equation to determine the improvement of a system when only a portion of the system is improved.
Additional Reference on Amdahl's Law

architectural registers: registers (Floating point and General Purpose) that are visible to the programmer.

ARF: architectural register file or retirement register file Additional Reference on Registers

Asynchronous Message Passing: a processor requests data, then continues processing instructions while message is retrieved. 

BHT: branch history table - records if branch was taken or not taken.

blocking cache: the cache services only one block at a time, blocking all other requests

BTB: branch target buffer - keeps track of what address was taken last time the processor encountered this instruction. 

cache: Additional Reference on Caches

cache coherence definition #1: Definition #1 - A read R from address X on processor P1 returns the value written by the most recent write W to X on P1 if no other processor has written to X between W and R. Additional Reference on Cache Coherence

cache coherence definition #2: Definition #2 - If P1 writes to X and P2 reads X after a sufficient time, and there are no other writes to X in between, P2’s read returns the value written by P1’s write. 

cache coherence definition #3: Definition #3 - Writes to the same location are serialized:two writes to location X are seen in the same order by all processors. 

cache hit: desired data is in the cache and is up-to-date

cache miss: desired data is not in the cache or is dirty

cache review: a cache review can be found at Additional Reference on Caches Additional Reference on Caches

cache thrashing: when two or more addresses are competing for the same cache block. The processor is requesting both addresses, which results in each access evicting the previous access.Additional Reference on Cache Thrashing

CDB: common data bus Additional Reference on the Common Data Bus

check pointing: store the state of the CPU before a branch is taken. Then if the branch is a misprediction, restore the CPU to correct state. Don't store to memory until it is determined this is the correct branch. 

CISC Processor: complex instruction set

CMP: chip multiprocessor

coarse multi-threading: the thread being processed changes every few clock cycles

coherence: [Additional Reference on Coherence[13]

computer architecture Additional Reference on Computer Architecture

consistency: order of access to different addresses Additional Reference on Consistency

control hazard: branching and jumps cannot be executed until the destination address is known

CPI: cycle per instruction

CPU: central processing unit

Dark Silicon: the gap between how many transistors are on a chip and how many you can use simultaneously. The simultaneous usage is determined by the power consumption of the chip. 

data hazard: the order of the program is changed which results in data commands being out of order, if the instructions are dependent - then there is a data hazard. 

DDR SDRAM: double data rate synchronous dynamic RAM

dependency chain: long series of dependent instructions in code Additional Reference on Dependency Chains

directory protocols: information about each block state in the caches is stored in a common directory.
Additional Reference on Directories

distributed caches: Additional Reference on Distributed Caches

DRAM: dynamic random access memory

DSM: distributed shared memory - all processors can access all memory locations

DSP Processor: a processor designed for digital signal processing tasks

ECC: error correct code

Enterprise class: used for large scale systems that service enterprises

error: defect that results in failure

error forecasting: estimate presence, creation, and consequences of errors

error removal: removing latent errors by verification

exclusion property: each cache level will not contain any data held by a lower level cache

explicit ILP: compiler decides which instruction to execute in parallel

failure: the cause of an error

fault avoidance: prevent an occurrence of faults by construction

fault tolerance: prevent faults from becoming failures through redundancy

faults: actual behavior deviates from specified behavior

FIFO: first in first out

fine multi-threading: the thread being processed changes every cycle

FLOPS: floating point operations per second

Flynn's Taxonomy: classifications of parallel computer architecture, SISD, SIMD, MISD, MIMD
Additional Reference on Flynn's Taxonomy

FPR: floating point register

FSB: front side bus

Geometric Mean: the nth root of the product of the numbers

global miss rate: (the # of L2 misses)/(# of all memory misses)

GPR: general purpose register

hit latency: time it takes to get data from cache. Includes the time to find the address in the cache and load it on the data lines

ILP: instruction level programming Additional Reference on ILP, ILP Ref #2, Additional ILP References

inclusion property: each level of cache will include all data from the lower level caches

IPC: instructions per cycle

Iron Law: execution time is the number of executed instructions N (write N in in the ExeTime for Single-Cycle), times the CPI (write x1), times the clock cycle time (write 2ns) so we get N2ns (write =N2ns) for single-cycle. 

Iron Law: instructions per program depends on source code, compiler technology, and ISA. CPI depends upon the ISA and the micro architecture. Time per cycle depends upon the micro architecture and the base technology. 

iron law of computer performance: relates cycles per instruction, frequency and number of instructions to computer performance

ISA: instruction set architecture

Itanium architecture: an explicit ILP architecture, six instructions can be executed per clock cycle

Itanium Processor: Intel family of 64-bit processors that uses the Itanium architecture

Lamport's Bakery Algorithm An algorithm to choose which process will be able to acquire the lock. Additional Reference

LFU: least frequently used

LLC last level cache

ll and sc: load link and store conditional, a method using two instructions ll and sc for ensuring synchronization.
Additional Reference on Synchronization

local miss rate: # of L2 misses/ # of L1 misses

locality principle: things that will happen soon are likely to be similar to things that just happened.

loop interchange: used for nested loops. Interchange the order of the iterations of the loop, to make the accesses of the indexes closer to what is actually the layout in memory

LRU: least recently used

LSQ: load store queue Additional Reference on LSQ

MCB: memory conflict buffer - "Dynamic Memory Disambiguation Using the Memory Conflict Buffer", see also "Memory Disambiguation" Additional Reference on MCB, Additional Reference on Memory Disambiguation

memory dependence prediction: Additional Reference on Memory Dependence Prediction

memory level parallelism: overlapping misses in a non-blocking cache leads to improved performance.

MOESI Protocol: modified-exclusive-owner-shared-invalid protocol, the states of any cached block.

MESI Protocol: modified-exclusive-shared-invalid protocol, the states of any cached block.

Message Passing: a processor can only access its local memory. To access other memory locations is must send request/receive messages for data at other memory locations. 

meta-predictor: a predictor that chooses the best branch predictor for each branch. 

MIMD: multiple instruction stream, multiple data streams

MISD: multiple instruction streams, single data stream

miss latency: time it takes to get data from main memory. This includes the time it takes to check that it is not in the cache and then to determine who owns the data, and then send it to the CPU. 

MLP memory level parallelism

mobo: mother board

Moore's Law: Gordon E. Moore observed the number of transistors on an integrated circuit board doubles every two years.

MP: multiprocessing

MPKI: Misses per Kilo Instruction

MSI Protocol: modified-shared-invalid protocol, the states of any cached block. 

MTPI: message transfer part interface

MTTF: mean time to failure

MTTR: mean time to repair

multi-level caches: caches with two or more levels, each level larger and slower than the previous level.
Additional Reference on Mulit-level Caches

mutex variable: mutually exclusive (mutex), a low level synchronization mechanism. A thread acquires the variable, then releases it upon completion of the task. During this period no other thread can acquire the mutex.  Additional Reference on Mutexes

NMRU: not most recently used

non-blocking caches: if there is a miss, the cache services the next request while waiting for memory

NUMA: non-uniform memory access, also called a distributed shared memory

OOO: out of order

OS: operating system

PAPT: physically addressed, physically tagged cache - the cache stores the data based on its physical address

PC: program counter

PCI: peripheral component interconnect

Pentium Processor: x86 super scalar processor from Intel Additional Reference on the Pentium

phantom exceptions: if a branch is mispredicted a number of instructions are executed before the misprediction is detected. It is possible to generate an exception within these executed instructions, this is a phantom exception, it should never have occurred.

physical registers: registers, FP and GP that are not visible to the programmer

pipeline burst cache: Additional Reference on Pipeline Burst Cache

pipelined cache: a pipelined burst cache uses 3 clock cycles to transfer the first data set from a cache block, then 1 clock cycle to transfer each of the rest. The pipeline and the 'burst'. (3-1-1-1) Reference on Pipelined Caches, Additional Reference on Pipelined Caches Additional Reference on Pipeline Caches

PIPT: physically indexed, physically tagged cache.

Power: Power = 1/2C V^2 * f Alpha

Power Architecture: performance optimization with enhanced RISC

Power vs Performance Equation: P ~ f^3 (assuming V ~ f, see Processor Speed, Power, Cost)

pre-fetch buffer: when getting data from memory, get all the data in the row and store it in a buffer.
Additional Reference on Pre-fetch Buffers

pre-fetching cache: instructions are fetched from memory before they are needed by the cpu "

Prescott Processor: Based on the Netburst architecture. It has a 31 stage pipeline in the core. The high penatly paid for mispredictions is supposedly offset with a Rapid Execution Engine. It also has a trace execution cache, this stores decoded instructions and then reuses them instead of fetching and decoding again. 

PRF: physical register file

pseudo associative cache: an address is first searched in 1/2 of the cache. If it is not there, then it is searched in the other half of the cache. Additional Reference on Caches

RAID: redundant array of independent disks

RAID 0: strips of data are stored on disks - alternating between disks. Each disk supplies a portion of the data, which usually improves performance.  Additional Reference on RAID

RAID 1: the data is replicated on another disk. Each disk contains the data. Which ever disk is free responds to the read request. The write request is written to one disk and then mirrored to the other disk(s). Additional Reference on RAID

RAID 2 and RAID 3: the data is striped on disks and Hamming codes or parity bits are used for error detection. RAID 2 and RAID 3 are not used in any current application

RAID 4: Data is striped in large blocks onto disks with a dedicated parity disk.  It is used by the NetApp company.

RAID 5: Data is striped in large blocks onto disks, but there is no dedicated parity disk. The parity for each block is stored on one of the data blocks.

RAR: read after read

RAS: return address stack

RAT: register alias table

RAT: register allocation table - Additional Reference on RAT

RAW: read after write

RDRAM: direct random access memory

relaxed consistency: some instructions can be performed ooo and still maintain consistency
Additional Reference on Relaxed Consistency

reliability: measure of continuous service accomplishment

reservation stations: function unit buffers

RETO: return from interrupt

RF: register file

RISC Processor: reduced instruction set - simple instructions of the same size. Instructions are executed in one clock cycle

ROB: re-order buffer

RS: reservation station

RWX: read - write- execute permissions on files

SHARC processor: floating point processors designed for DSP applications Additional Reference on the SHARC Processor

SIMD: singe instruction stream, multiple data streams

simultaneous multi-threading: instructions from different threads are processed, even in the same cycle

SISD: single instruction stream , single data stream

SMP: symmetric multiprocessing

SMT: simultaneous multi threading

snooping protocols: A broadcast network - caches for each processor watch the bus for addresses in their cache. 

SPARC processor: Scalable Processor Architecture -a RISC instruction set processor

spatial locality: if we access a memory location, nearby memory locations have a tendency to be accessed soon. 

Speedup: how much faster a modified system is compared to the unmodified system.

SPR: special purpose registers - such as program counter, or status register

SRAM: static random access memory

structural hazard: the pipeline contains two instructions attempting to access the same resource.

super scalar architecture: the processor manages instruction dependencies at run-time. Executes more than one instruction per clock cycle using pipelines.

synchronization: "a system is sequentially consistent if the result of any execution is the same as if the operations of all the processors were executed in some sequential order, and the operations for each individual processor appear in the order specified by the program." Quote by Leslie Lamport

Synchronous Message Passing: a processor requests data then waits until the data is received before continuing.

tag: the part of the data address that is used to find the data in the cache. This portion of the address is unique so that it can be distinguished from other lines in the cache. 

temporal locality: if a program accesses a memory location, it tends to access the same location again very soon. 

TLB: translation look aside buffer - a cache of translated virtual memory to physical memory addresses. TLB misses are very time consuming Additional Reference on Look Aside Bufferes

Tomasulo's Algorithm: achieve high performance without using special compilers by using dynamic scheduling
Additional Reference on the Tomasulo algorithm, Additional Reference for Tomasulo's Algorithm

tournament predictor: a meta-predictor

trace caches: sets of instructions are stored in a separate cache. These are instructions that have been decoded and executed. If there is a branch in the set, only the taken branch instructions are kept. If there is a misprediction the trace stops. Reference on Trace Caches

trace scheduling: rearranging instructions for faster execution, the common cases are scheduled first Additional Reference on Static ILP, Reference on Trace_scheduling

tree, tournament, dissemination barriers: types of structures for barriers
Additional Reference on Barriers

UMA: uniform memory access - all memory locations have similar latencies

vector registers: hold data for vector processing for SIMD

victim cache: a cache that holds recently evicted blocks.  The link is to a victim simulator (don't use the first trace program, need Firefox, java7).  Additional Reference on Victim Caches

VIPT: virtually indexed physically tagged

VIPT: virtually indexed physically tagged - the cache receives the virtual address as does the TLB (in parallel). If a hit, okay, if a miss, the physical address is already translated.  Additional Reference on Virtual Memory

virtual memory: a method of allowing a process to access RAM memory independently of other processes. RAM memory is used by all processes, which means there is probably not enough RAM for all processes at the same time. The virtual address is used to point to an address, either RAM or Disk. The program sees it as the same, the TLB translates between request and the actual memory.  Additional Reference on Virtual Memory

VIVT: virtually indexed virtually tagged

VIVT: virtually indexed, virtually tagged cache - the cache is virtually addressed by the cpu. On a cache miss the virtual address must be translated to the physical address

VLIW: very long instruction word

VPN/PPN: Virtual Physical Network, Physical page network

WAR: write after read

WAW: write after write

way prediction: set associative caches are fast but use a lot of energy. To save energy, predict the address that will be selected. This reduces the energy wasted searching the entire cache.  Additional Reference on Way Prediction

write invalidate: a write to shared data forces invalidation of all other cached copies

write update: a write to shared data is broadcast to update copies

XEON: x86 processors that can operate together to form dual and quad core systems