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MIPS R4300i Microprocessor

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R4300i MICROPROCESSOR 


MIPS R4300i Microprocessor 
Technical Backgrounder -Preliminary


Chapter 1. R4300i Technical Summary 


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Performance: 			SPECint92 	60 
				SPECfp92 	45

ISA Compatibility 		MIPS-I, MIPS-II, AND MIPS-III


Master Clock Frequency 	10 Mhz min/ 67 Mhz max


Pipeline Clock 		10 Mhz min/ 100 Mhz max (1x, 1.5x, 2x, or 3x of master clock)

System Interface clock 	67 Mhz max

Caches 			16 KB I-cache and 8 KB D-cache

TLB 				32 double entries; Variable Page size (4 KB to 16 MB in 4x increments)

Power dissipation: 		1.8 watts (typ.) at max. operating frequency

Supply voltage 		min 3.0 V typ. 3.3 V max 3.6 V

Packaging: 			120-pin Plastic Quad Flat Pack (PQFP)

Die size: 			44 mm2 

Process Technology: 		0.35 micron, 3-level-metal CMOS


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Chapter 2. Overview 


This paper introduces the RISC R4300i microprocessor from MIPS Technologies,
Inc. (MTI). The information presented in this paper discusses how 
the R4300i differs from previous microprocessors from MTI. 

This chapter provides general information on the R4300i, including: 


ےےے[*]Introduction 
ےےے[*]The R4300i microprocessor 
ےےے[*]Packaging and design support 
ےےے[*]Future upgrades 


Introduction 


Reduced instruction-set computer (RISC) architectures differ from 
older complex instruction-set computer (CISC) architectures by optimizing 
performance for the available silicon area. The MIPS architecture,
developed by MTI, is firmly established as the leading RISC architecture 
today. 

The MIPS R4300i microprocessor extends the benefits of RISC's performance 
to consumer electronics. The R4300i microprocessor also delivers 
high performance to existing embedded and computing applications 
at a low cost. The low cost and high performance provided by the 
R4300i are needed for the latest consumer applications such as interactive 
television and games. 

In the beginning, RISC microprocessors were typically used for high 
performance applications. Lately, these processors have found their 
way into the embedded systems market as well. Today, MIPS RISC processors 
are used in network controllers, laser printers, and X-terminals 
among other applications. The migration of MIPS RISC processors to 
these applications has been facilitated by lower costs as well as 
high integration of various functional blocks into a single die. 
The R4300i can deliver up to 60 times the integer performance of 
a VAX 11/780 (60 SPECint '92) at a cost approaching less than $1 
per SPECint. The R4300i is also designed for low power so that it 
can be offered in a low cost plastic package. This makes the R4300i 
a strong candidate for consumer and embedded applications. 

Between 1985 and 1994, three generations of the MIPS architecture 
have been introduced and widely adopted. The first commercial MIPS 
processor, the R2000, ran at 8-MHz and used a 32-bit architecture. 
The R3000 family raised system speed to 40 MHz. The R4000 family 
uses a 64-bit architecture to boost instruction throughput and increase 
the available address space. It also adds multilevel cache and multiprocessor 
capabilities. The R4000 family (R4400PC, R4400SC, R4400MC, R4000PC 
and R4000SC) currently work at pipeline speeds of up to 200 MHz. 
Recently MTI announced the MIPS R10000 microprocessor that offers 
industry leading performance for scientific and database applications. 


MTI's semiconductor partners have successfully implemented MIPS standard 
processors in a variety of semiconductor processes and introduced 
numerous derivative products. MTI semiconductor partners include 
Integrated Device Technology, Inc., LSI Logic Corp., NEC Corporation,
Performance Semiconductor, Inc., Siemens and Toshiba Corporation. 
Users of the MIPS architecture include AT&T, Cisco, Control Data,
NEC, Network Computing Devices, Pyramid Technology, QMS, Siemens-
Nixdorf, Silicon Graphics, Sony, Texas Instruments and Tektronix. 

R4300i Microprocessor 


The R4300i uses a variety of techniques to provide high performance 
at low cost and low power consumption. These techniques include power-
reduction features, power management features, cost-reduction features,
and architectural optimizations. 

Major R4300i characteristics include 64-bit processing, 100-MHz internal 
pipeline clock frequency, low-voltage operation, power-saving modes,
plastic packaging, and a single data path for integer and floating-
point operations. The R4300i implements the MIPS-III instruction 
set architecture and is fully software compatible with all existing 
MIPS processors. Chapter 3 provides a complete description of architectural 
enhancements in the R4300i over previous MIPS microprocessor families 
and other R4000 family members. 
R4300i Family


The R4300i is the first member of a family of microprocessors. The 
R4300i family uses high integration, power management and virtual 
memory implementation to bring high performance and low cost to the 
consumer market. Future R4300i family members are planned to increase 
the options available to systems developers. 

The R4300i has been designed in well-defined basic blocks to simplify 
implementation of the R4300i core logic in new products. For example,
by removing the caches, memory-management unit, and system interface,
a high-performance RISC core is available for integration in a derivative 
processor or ASIC. 
Packaging and Design Support


The R4300i will be made available in a single 120-pin PQFP package. 


The 120-pin plastic quad flat pack (PQFP) offers a low-cost package 
for surface-mount assembly, decreasing processor cost further to 
benefit embedded applications, and with a low profile suitable for 
consumer applications. 
Future Upgrades


It is anticipated that higher frequency versions of the R4300i will 
become available in the future. These will provide an extra performance 
boost at the same low price points as the current R4300i. 
Chapter 3. Implementation 


The R4300i differs from the R4000 family in four main categories. 
These categories, discussed in the next sections, are:- 


ےےے[*]Power reduction features 
ےےے[*]Power management features 
ےےے[*]Cost reduction features 
ےےے[*]Architectural optimization 
ےےے[*]System bus interface 


These features combine to achieve typical power dissipation of 1.8 
watts, a reduced power mode dissipation of 0.4 watts, and a power-
down mode where the processor is turned off. These features also 
allow a die size of less than 7mm on a side, while maintaining full 
64-bit operation. 
Power Reduction Features


The R4300i is designed using low-power design techniques. These are 
techniques that reduce power dissipation while running standard tasks. 


Examples of low-power design techniques, discussed below, are: 


ےےے[*]3.3-Volt operation 
ےےے[*]Dynamic logic design 
ےےے[*]Cache bank partitioning 
ےےے[*]Write-back data cache 
ےےے[*]Cache prefetching 
ےےے[*]Micro TLB 


3.3-Volt operation


The R4300i was designed for operation at 3.3V to reduce power consumption. 


CMOS power dissipation increases with the square of potential difference 
between power (VDD) and ground (VSS). At lower voltage levels, however,
the threshold voltage at which a logic signal switches between zero 
to one changes. Redesign of the gate's physical width-to-length ratio 
is necessary to maintain logic speed at lower voltages; that slightly 
increases total power dissipation. In sum, reducing the rail-to-rail 
voltage difference by 1.7 volts reduces overall comparative power 
dissipation to about 70%. 

A lower rail-to-rail (VDD to VSS) voltage difference also increases 
the chip's sensitivity to electrical noise, as there is a smaller 
noise margin around the threshold voltage. MTI designed the R4300i 
interface using low-voltage CMOS (LVCMOS) characterization to ensure 
noise immunity and reliable signal operation. 
Dynamic logic design


The R4300i uses dynamic rather than static logic design to reduce 
transistor count and power dissipation. The R4300i 's power management 
functions make the dynamic logic design approach suitable for use 
in consumer applications. 
Cache bank partitioning


Most instruction and data cache accesses exhibit spatial and temporal 
locality of reference. In the R4300i, the instruction and data caches 
are each split into four banks; only one of the four banks in the 
instruction or data caches is powered up at any one time. This saves 
power on every cache access cycle. 

There is no performance degradation on cache bank misses; enabling 
of cache banks is entirely transparent to system operation. 
Write-back cache


The R4300i uses a write-back policy for write operations. In a write-
back policy, data in the cache is written to the main memory only 
when the cache line is replaced. Cache lines can be replaced whenever 
new data from a different address is to be loaded into the cache 
or if the cache line is invalidated. 

A write-back cache policy reduces store activity on the system bus,
which improves system performance and simplifies memory subsystem 
design. 
Cache prefetching


As instruction reads are usually sequential, the R4300i fetches two 
consecutive 32-bit words (instructions) every time it reads the instruction 
cache. Dual instruction access from the cache reduces the frequency 
of cache enabling for instruction access, which reduces power dissipation. 

TLB and Micro Instruction-TLB


The memory management unit (MMU) translates virtual addresses to 
physical addresses by looking up address correspondences in a "page 
table". The processor maintains a complete page table in main 
memory, but accesses to main memory are slow. The translation lookaside 
buffer (TLB) keeps copies of page table entries on-chip, which accelerates 
virtual-to-physical address translation. 

The TLB structure is large and consumes power when enabled. The R4300i 
therefore includes a "micro TLB" on chip which contains 
page table entries for the two most recently referenced instruction 
pages. 
Power Management Features


The R4300i also has built-in power management features in addition 
to power reduction features. Power management is used when peak performance 
is not required and it allows the processor to operate in different 
modes. These modes require less power and therefore reduce average 
power consumption over a period of time. The power management modes 
, discussed below, are:- 


ےےے[*]Standard operating mode 
ےےے[*]Sleep mode 
ےےے[*]Power-down mode 


Standard operating mode


In this mode the processor operates at a maximum of 100 MHz pipeline 
speed and 50 MHz external interface speed. Power dissipation at maximum 
frequency in this mode is estimated at 1.8 watts. 
Sleep mode


This feature allows the processor to change dynamically to one quarter 
of the normal speed. For example, if the pipeline operates normally 
at 100 Mhz, it would operate at 25 Mhz in sleep mode. Typically, 
chipset logic triggers this mode when it detects no user activity 
over some pre-determined amount of time(such as between keystrokes 
or mouse movements). In reduced power mode, power dissipation falls 
to 0.4 Watts, one quarter of the normal. 
Power-down mode


In the R4300i, all variable registers are both readable and writable. 
On power-down, the state of the processor can therefore be written 
to non-volatile RAM. On power-up, the registers can be restored to 
the same state. 

This "instant-on" capability allows the processor to be 
activated in milliseconds, instead of the many seconds normally required 
to boot an operating system. This not only reduces power consumption 
but is a power saving benefit for consumers. 
Cost Reduction Features


The R4300i is designed for low cost. The main areas contributing 
to microprocessor cost are packaging, test and assembly, and die 
costs. 
Packaging cost reduction


For low cost applications, the R4300i will be available in a 120-
pin plastic quad flat package (PQFP). High-performance microprocessors 
normally require expensive ceramic or metal packages with enhanced 
thermal dissipation because of their higher power requirements. 
The reduction in the system bus width from 64 bits to 32 bits and 
elimination of some control signals resulted in a low pin-count package 
offering. Lower power dissipation facilitated the use of plastic 
packages. 
Test & assembly cost reduction


The R4300i implements column redundancy in both instruction and data 
caches. Column redundancy reduces the chip's sensitivity to defects 
in the caches. 

The test process first tests the integrity of each bit column in 
the cache; polysilicon fuses in the chip are then blown to swap redundant 
bit columns for defective bit columns. 

Column redundancy increases the die yield at the test stage, which 
in turn decreases testing costs for each good die. 
Die cost reduction


The die area was reduced by the following techniques: 


ےےےHigh-density 0.35 micron design rules 
ےےےUse of 4-transistor RAM cells in the caches 
ےےےUnified CPU/FPU data path 
ےےےReduced configurability 
ےےےOptimized cache and TLB size 


The last three cost reduction strategies fall in the category of 
architectural optimizations, discussed below. 
Architectural Optimizations


The R4300i fully implements the current ISA, MIPS-III standard. 

As in all the MIPS R3000 and R4000 processors, an on-chip CP0 coprocessor 
contains an MMU for virtual address translation and exception processing 
control. The system address/data bus interface is different from 
the R4000 series processors. The R4300i has a 32-bit multiplexed 
address/data bus and a 5-bit command bus. A flush buffer, which 
was added to the R4400 for increased graphics performance, is retained 
in the R4300i. 

The R4300i also includes on-chip clock frequency division circuitry 
to support internal 100-MHz operation from an external 50-MHz clock. 
The R4300i has the option of operating internally at 1, 1.5, 2 or 
3 times the frequency of the external clock. In addition, the R4300i 
eliminates RClock that existed in the R4000. The output clocks SyncOut 
and TClock can be turned off for power savings. This allows high 
microprocessor performance and also simplifies system design. 

The differences from the R4000, discussed below, are: 


ےےے[*]Combined integer/floating-point data path 
ےےے[*]Optimized 5-stage pipeline 
ےےے[*]Optimized cache and TLB size 
ےےے[*]Reduced physical address space 
ےےے[*]Additional instruction trace support 
ےےے[*]Simplified processor initialization 


Unified integer/floating-point data path


The R4300i's integer unit shares its data path with the FPU unit. 
CPU and floating-point instructions are both executed in the same 
5-stage pipeline. This represents a considerable saving in die area 
and corresponding power dissipation. 
Optimized pipeline


Pipelining allows multiple instructions to overlap during execution 
for greater throughput. All processor operations require five basic 
operations: instruction fetch, instruction decode, instruction execution,
accessing data and writing of the results. These operations can be 
split up over a pipeline so that several instructions can be treated 
concurrently: while one instruction is being decoded, another can 
be fetched, and so on. 

The R4300i uses a five-stage pipeline instead of the eight-stage 
pipeline found in the R4000 series processors. The eight-stage pipeline 
allows the R4000 series processors to reach higher speeds. However,
the shorter pipeline achieves greater efficiency at a given speed 
than its longer counterpart. Loads, stores, jumps and branches are 
resolved in fewer cycles, and exception processing is simplified. 
Less control logic means reduced die area. 
Optimized cache and TLB size


The R4300i uses separate instruction and data caches. Instruction 
cache size impacts performance to a greater extent owing to locality 
of instruction code. The combination of 16-Kbytes instruction cache 
and 8-Kbytes data cache gives optimum performance for a fixed total 
cache size. The on-chip TLB is also reduced from the 48 entry-pairs 
in the R4000 to 32 entry-pairs. These sizes have been selected after 
extensive simulation to give the R4300i the best trade-off between 
high performance and small die area. TLBs are very critical in implementing 
virtual memory systems. In consumer applications such as settops,
the existence of TLBs allow for implementation of security as well 
as fast context switching times when running multiple processes. 
In addition, virtual page sizes from 4KB, 16KB, 64KB, 256KB, 1MB,
4MB, and 16MB are supported just as in the R4000 series processors. 

Reduced physical address space


The physical address space has been reduced from 36 bits to 32 bits. 
This still supports a physical address range of 4 Gbytes, more than 
enough for consumer applications. 
Additional instruction trace support


The R4300i includes an additional debugging mode called instruction 
trace support. This mode lets the user find out the physical address 
to which the CPU has branched or jumped whenever a branch, jump, 
or exception is taken. 
Simplified processor initialization


Processor initialization has been simplified, reflecting the lower 
degree of configurability. The R4300i has two hardware pins for configuration 
during the reset initialization sequence and a configuration register 
in CP0 is used to set other options such as data rate or endianess. 

Chapter 4. Benefit Summary


The main benefits of the R4300i, discussed below, are: 


ےےے[*]Price/Performance 
ےےے[*]Low Power 
ےےے[*]Low Cost 
ےےے[*]Compatibility 


Price/Performance


The R4300i dramatically improves price/performance for both system 
designers and end-users. 

The R4300i achieves price/performance over ten times better than 
existing microprocessors. The R4300i will be the first commercially 
available processor to deliver less than $1/SPECint mark in 1995. 

Low Power


The R4300i was specifically designed for high-performance consumer 
applications by including reduced power and power management features. 


ےےےReduced-power features are those that perform traditional tasks 
ےےےin a way that reduces power consumption. 
ےےےPower management features are architectural enhancements that 
ےےےfurther reduce internal and system-wide power consumption through 
ےےےswitching modes. 


The combination of reduced-power and power-management features allows 
the R4300i to fit in an inexpensive plastic package. 
Low Cost


In practice, good price/performance usually means increased performance 
at a given price point. The R4300i, in contrast, brings existing 
high-performance levels to an unprecedented low price-point. 

System manufacturers can decrease component costs, thereby increasing 
margins, or offer their products at a lower price, thereby stimulating 
demand. 

Price-sensitive consumer applications such as games and interactive 
television benefit particularly from a low cost, high-speed microprocessor. 
Factory automation and robotics are also likely applications for 
the R4300i. 
Compatibility


Software compatibility is a fundamental requirement to preserve investments 
in software over time. 

All MIPS processors maintain software compatibility. A program compiled 
and linked to run on the R3000 processor will run on the R4300i. 


Appendix A. Glossary


Cache. An on-chip temporary storage area containing a copy of main 
memory fragments. Cache access is much faster than main memory access. 


CISC (Complex Instruction Set Computing). A design approach that 
attempts to achieve performance gains with complex instruction and 
data types and hardware controlled memory management. 

CPU (Central Processing Unit). The part of a microprocessor where 
the majority of the instructions are executed. 

Die. The silicon chip after it has been cut from a wafer and before 
it has been packaged. 

Flush Buffer (also called a write buffer). The flush buffer is a 
temporary storage location for data that is being written from the 
pipeline or cache to main memory. The flush buffer allows the processor 
to continue executing instructions while data is being written to 
main memory. 

FPU (Floating-Point Unit). Dedicated logic to accelerate calculations 
using floating-point numbers. 

IU (Integer Unit). The part of a CPU that performs calculations using 
integer arithmetic. 

LVCMOS (Low-voltage CMOS). An IEEE standard for low-voltage logic 
design. 

MMU (Memory Management Unit). That part of a microprocessor which 
implements virtual-to-physical address translation and the memory 
system hierarchy including cache memory. 

MTI (MIPS Technologies, Inc.). The developer of the MIPS RISC architecture,
the leading RISC architecture worldwide. 

Page Table. An area of main memory containing sets of virtual addresses 
with their corresponding physical addresses and protection data. 


RISC (Reduced Instruction Set Computing). A design philosophy that 
avoids implementing complex functions in silicon but realizes large 
performance increases through executing simpler, standardized instructions 
at faster, more efficient rates. 

Pipeline. A mechanism to allow multiple instructions to overlap during 
execution for greater throughput. A five-stage pipeline offers peak 
performance five times that of a non-pipelined processor. 

PQFP (Plastic Quad Flat Pack). A plastic package with pins on the 
four edges, cheaper than a CPGA. 

TLB (Translation Lookaside Buffer). An on-chip "page table" 
cache containing copies of the page tables used by the MMU for virtual-
to-physical address translation. 

64-bit Processor. A processor in which all address and data paths 
are 64 bits wide. Leading-edge applications require 64-bit processors 
today. 32-bit capability in a 64-bit processor is important to manage 
the smooth transition from 32 to 64 bits. The MIPS R4000, R4400 and 
R4300i MPUs can run in either 32-bit or 64-bit mode. 
Appendix B. Bibliography


The following related documents are available from MIPS Technologies,
Inc. 

MIPS R4400 Microprocessor, Technology Backgrounder. 

MIPS Technologies, Inc., Corporate Backgrounder. 

NEC Corporation, Corporate Backgrounder. 

R4000 / R4400 User's Manual, [Prentice Hall]. 

MIPS RISC Architecture, Kane & Hall [Prentice Hall]. 

Contact MIPS Technologies, Inc. for a more complete list of publications 
available on RISC technology and the MIPS architecture. 

MIPS Technologies, Inc. Information Service: 

1-800- I GO MIPS or 1-800-446-6477 Inside the US 

415-688-4321 Outside the U.S 

World Wide Web: URL to [*]http://www.mips.com 

MIPS, the MIPS Technologies logo, and R3000 are registered trademarks,
and R2000, R4000, R4400, and R6000 are trademarks of MIPS Technologies,
Inc. 

Windows NT is a trademark of Microsoft Corp. 

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