What are the differences between Level 1 and Level 2 cache on the Power Macintosh computers? Do all Power Macintosh computers use the same type of L2 cache?
Level 1 Cache consists of high speed memory built into the PowerPC processor. By using this cache, the processor can access frequently-requested data more quickly. The amount of Level 1 cache varies among the PowerPC chips, and you cannot upgrade it.
The PowerPC 601 microprocessor and the PowerPC 604 microprocessor both have 32K internal cache. However, the PowerPC 601 microprocessor has a single 32K cache for both instructions and data, whereas the PowerPC 604 has two 16K sections, one for instructions and one for data. The PowerPC 604e microprocessor has 64K cache, which consists of two 32K sections, one each for instructions and data.
Begin_Table
Computer L1 Cache
------- --------
Power Macintosh 5200/75 & Performa 5200 Series 16K
Power Macintosh 5260/100, 5300/100 & Performa 5300 Series 32K
Macintosh Performa 6200 Series 16K
Macintosh Performa 6290CD and 6300 Series 32K
Macintosh Performa 6400 Series 32K
Macintosh Performa 6360/160 32K
Power Macintosh 5400 Series 32K
Power Macintosh 6400 Series 32K
Power Macintosh & Performa 6100 Series 32K
Power Macintosh 7100 Series 32K
Power Macintosh 8100 Series 32K
Power Macintosh 7200 Series 32K
Power Macintosh 7500/100 32K
Power Macintosh 7600 Series 32K
Power Macintosh 8500/120 32K
Power Macintosh 8500/132 32K
Power Macintosh 8500/150 32K
Power Macintosh 8500/180 64K*
Power Macintosh 9500/120 32K
Power Macintosh 9500/132 32K
Power Macintosh 9500/150 32K
Power Macintosh 9500/180MP 64K*
Power Macintosh 9500/200 64K*
End_Table
*These Power Macintosh computers are currently the only products with the PowerPC 604e microprocessor.
Level 2 Cache
=============
Level 2 Cache is separate from the processor and it is typically upgradeable. The Level 2 cache works in conjunction with the microprocessor's internal cache to provide maximum performance. The total amount of supported Level 2 Cache also varies from computer.
Performance Benefits
--------------------
* Native PowerPC Software
You see the greatest performance improvement in tightly written native PowerPC software where code is kept close to the microprocessor.
* Non-Native Software
You will likely see marginal or no performance improvements running software applications written for 68000-series microprocessors.
PowerPC Microprocessors
-----------------------
The reason for performance increases with the addition of L2 cache is the PowerPC microprocessor can keep its pipeline full, allowing for faster and more efficient processing. The microprocessor first checks its internal cache, then L2 cache, and finally main memory (DRAM) for instructions. Because cache memory is faster than DRAM, it can be accessed more quickly, thus helping keep the pipeline full.
This also accounts for the lack of uniform performance improvements. Tight code will stay near the processor and run faster, while other code may not. In general, you can expect a 10-15% performance improvement with code that benefits from L2 cache.
Power Macintosh Models
----------------------
The Nubus-based Power Macintosh computers (which include the 6100, 7100, and 8100 series) and the PCI-based Power Macintosh computers (which include the 7200, 7500, 8500, and 9500 series) do not use the same type of L2 cache.
* Power Macintosh 6100, 7100, and 8100
The 6100, 7100 and 8100 Power Macintosh models accept an external L2 cache on a Single Inline Memory Module (SIMM). The Power Macintosh 8100/80 shipped with 256K L2 cache already installed; users of Power Macintosh 6100/60, 6100/60AV, 7100/66, and 7100/66AV can install a L2 cache by installing the appropriate cache SIMM into the 160-pin connector on the main logic board.
The Power Macintosh 6100/66, 7100/80, 8100/100, and 8100/110 computers ship with 256K L2 cache memory installed.
* Power Macintosh 7200, 7500, 8500, and 9500
The Power Macintosh 7200, 7500, 7600, and 8500 series all support L2 cache 160-pin Dual Inline Memory Modules (DIMMs). The cache can be increased by adding L2 cache DIMMs to the expansion slot(s) on the logic board.
The Power Macintosh 7200/75, 7200/90, 7200/120 (8 MB configuration), and the 7500 series computers ship with no L2 cache DIMM installed.
The Power Macintosh 7200/120 (16 MB configurations), 7600 series, and the 8500 series include a 256K L2 cache DIMM installed. The Power Macintosh 7200 is theoretically expandable to 1 MB, while the 7500, 7600, and 8500 series computers are theoretically expandable to 4 MB, but those configurations have not been tested and are not supported by Apple.
The Power Macintosh 9500 series computers ship with 512K L2 cache memory soldered to the logic board. The Power Macintosh 9500 series computers do not have a DIMM socket for adding additional L2 cache memory, and therefore are not expandable beyond 512K.
7600 Series 256K DIMM 160-pin DIMM 256K,512K,1 MB** 1 MB**
8500 Series 256K DIMM 160-pin DIMM 256K,512K,1 MB** 1 MB**
9500 Series 512K On logic board N/A 512K
End_Table
*These computers theoretically support 512K and 1 MB of L2 cache, but these configurations have not been tested and are not supported by Apple.
** These computers support a theoretical limit of 4 MB of L2 cache, but this has not been tested and is not supported by Apple.
Notes:
The Power Macintosh 7200/120 configuration with 8 MB of memory does NOT include the L2 Cache DIMM.
This article is one of many available through the Apple Fax center. For a complete list of available Fax documents, search the Tech Info Library for Apple Fax Document Index or call the Apple Fax line at 1-800-505-0171 and select document number 20000 (Apple Fax - Document Index - Product Support Literature). The Apple Fax center is available free of charge 24 hours a day, 7 days a week.
This article was published in the Information Alley on 7 August 1996.
