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Power Mac Apple FAQs

Note: The majority of this information is taken directly from the Developer Technical Support TECHNOTE: SIMMs to DIMMs: Making Sense Out of Memory Expansion for the Power Macintosh.

  1. Why has Apple transitioned memory expansion from 72-pin SIMMs to 168-pin DIMMs for Power Macintosh computers?
  2. Why do SIMMs need to be installed in complementary pairs, when DIMMs can be added on a per module basis?
  3. How does the PowerPC address memory?
  4. What is memory interleaving and what advantage does it provide?
  5. How is memory interleaving enabled?
  6. What speed SIMMs and DIMMs are required for Power Macintosh computers?
  7. Can faster speed memory modules be used in Power Macintosh computers and will they increase system performance, for example, 60 nsec instead of 70 nsec modules for a performance increase of 10 - 15 percent?
  8. Can different speed memory modules be intermixed, for example, 60 and 70 nsec modules in a Power Macintosh computer that is specified at 70 nsec?
  9. What does refresh rate or count refer to (such as, 1K, 2K, or 4K)?
  10. How do Power Macintosh computers provide continuous refreshing?
  11. How are different sized SIMMs and DIMMs -- 16 Mbytes, 32 Mbytes, etc. -- designed with different types of DRAM devices, such as 4M x 4, 2M x 8, and 1M x 16 parts?
  12. Do Macintosh computers support composite memory modules?
  13. Can memory modules with different refresh rates or counts be intermixed?
  14. What is EDO memory and do Macintosh computers support it?
  15. What is parity memory and do Power Macintosh computers support it?
  16. What is ECC memory and do Macintosh computers support it?
  17. My manual suggests that when upgrading my cache, I use Apple-branded cache. I would like to purchase a 512k cache module, but it does not appear that Apple sells this size. What size should I purchase?





  1. Why has Apple transitioned memory expansion from 72-pin SIMMs to 168-pin DIMMs for Power Macintosh computers?

    New Macintosh computers incorporating DIMMs provide a number of key advantages for an ever-increasing RAM footprint and memory-hungry applications (such as in multimedia development):
    * a wider 64 bit data path
    * higher memory capacity
    * greater flexibility because systems do not require DIMMs to be installed in complementary pairs.
  2. Why do SIMMs need to be installed in complementary pairs, when DIMMs can be added on a per module basis?

    The reason is because of differences in the "width" of the data bus. On SIMMs, the data bus is 32 bits wide while the data bus on DIMMs is 64 bits wide.

    To span the PowerPC 64 bit data bus, SIMMs need to be installed in complimentary pairs . DIMMs, with their 64 bit data bus, can be installed individually in second generation Power Macintosh computers.

    The only exception to this are some entry-level Power Macintosh computers, such as the Power Macintosh 5200/75 LC, which incorporate a PowerPC 603 processor configured with a 32 bit bus. In such computers, SIMMs are used for memory expansion and not installed in identical pairs.
  3. How does the PowerPC address memory?

    PowerPC microprocessors are 32 bit processors that have 4 Gbytes of address space. This means that the processor instructions are encoded in 32 bits, and that they can address or access 4 Gbytes of data. Although the PowerPC has 4G of address space, only a portion of the 4G is allocated for main memory (DRAM memory). For example, the total RAM expansion on entry-level Macs can be as much as 136 Mbytes, while RAM expansion on Power Mac 9500 can be as much as 1.5G. The remaining portion of the address space is allocated for system ROM, PCI cards, system control, etc.

    Data is transferred between the PowerPC and main memory via the 64 bit data bus by memory reads or memory writes.

    There are two types of memory transactions: single- and four-beat transfers. A single-beat memory transaction reads or writes between one to 8 bytes of data. (Equating 8 bytes to 64 bits and noting that the data bus is 64 bits wide, the PowerPC can read or write eight bytes in a single memory operation.) A four-beat memory transaction reads or writes a cache block of 32 bytes to or from memory. Each of the four beats encompasses 8 bytes.

    What's important to understand is that the memory bus is 64 bits wide and up to 8 bytes can be transferred in a single memory operation.
  4. What is memory interleaving and what advantage does it provide?

    Even though the system data bus is 64 bits wide, the memory controller in Power Macintosh 7300, 7500, 7600, 8500, 8600, 9500, and 9600 computers can support 128 bit data read and write operations by interleaving data between corresponding DIMMS.

    Memory interleaving provides higher bandwidth (MBytes per second) between the PowerPC microprocessor and main memory. It also provides a significant performance boost, increasing the execution speed of memory-intensive programs. How much faster depends on the program's software architecture and whether an L2 cache is present.
  5. How is memory interleaving enabled?

    Memory interleaving is a function of the memory controller used in Power Macintosh 7300, 7500, 7600, 8500, 8600, 9500, and 9600 computers. Memory interleaving is enabled by the power-up software when it detects two DIMMs in corresponding expansion slots (such as, A1 and B1, A2 and B2, and so on) that are the same density, have the same memory bank configuration, and have the same DRAM addressing modes.
  6. What speed SIMMs and DIMMs are required for Power Macintosh computers?

    The access time of DRAMs used to expand memory on most Power Macintosh computers are 80 nsec, 70 nsec, and faster, as we move into the future. Always be sure to check memory expansion specifications for your particular computer before making a purchase.
  7. Can faster speed memory modules be used in Power Macintosh computers and will they increase system performance, for example, 60 nsec instead of 70 nsec modules for a performance increase of 10 - 15 percent?

    Faster speed memory modules will most likely work fine in Power Macintosh computers, for example, 60 nsec instead of 70 nsec modules. This will not increase CPU performance, however. The PowerPC processor will not access memory any faster with faster memory module speed, since Power Macintosh computers do not use memory module speed sense lines.

    Note: Apple recommends that for memory expansion that you stay with the speed specified for the computer. Although faster devices most likely will work, keep in mind that these are untested configurations.
  8. Can different speed memory modules be intermixed, for example, 60 and 70 nsec modules in a Power Macintosh computer that is specified at 70 nsec?

    As long as memory modules speeds are equal to or less than the Power Macintosh-specified memory speed, installing different speed modules should have no impact on the Power Macintosh computer's functionality.
  9. What does refresh rate or count refer to (such as, 1K, 2K, or 4K)?

    The term "4K refresh," for example, refers to the number of refresh cycles required to refresh all sections of the memory array and is determined by the number of row addresses. (12 addresses will address 4096, or 4K, locations. Similarly, 11 addresses will address 2048, or 2K, locations.)

    Data stored in DRAM devices is volatile. To maintain data integrity, DRAM requires a power source and continuous refreshing.

    DRAMs are specified at different refresh rates that may or may not be compatible with a particular PC model. Macintosh computers are compatible with widely available DRAM devices.

    For example, below are three common 16 MBit DRAM devices used to construct SIMMs or DIMMs.
    . Organization Refresh-Addressing Refresh-Addressing
    4M x 4 4K - 12/10 2K - 11/11
    2M x 8 4K - 12/9 2K - 11/10
    1M x 16 4K - 12/8 1K - 10/10

    .
    This information demonstrates that 4M x 4 devices come in 4K and 2K refresh rates, with 12/10 and 11/11 addressing, respectively. Addressing notation refers to a matrix array organization where 4K refresh devices (12/10) have 12 row addresses and 10 column addresses. Likewise, 2K refresh devices (11/11) have 11 row addresses and 11 column addresses. Both modes (12/10 & 11/11) have 22 address lines (number of column plus row lines) which is required for an address space of 4,194,304 (or 4M) locations, each of which contains a 4-bit word.

    Note: 4M x 4 refers to a DRAM device with an address space of 4M locations, each containing a data word 4 bits wide. (4M x 4 equals 16 Mbits; DRAM devices are usually referred to by size [total number of bits].)
  10. How do Power Macintosh computers provide continuous refreshing?

    The power source and refreshing are provided as long as the computer is powered on. The Memory Controller in Apple Macintosh and Power Macintosh computers generate a CAS (Columb Address Strobe) before RAS (Row Address Strobe) refresh cycle every 15.6 msec. DRAM devices contain an address refresh counter which is triggered by a CAS before RAS memory cycle.

    A 16 MBit (11/11) device with an 11 row address - referred to as a 2K refresh part - requires all row address combinations (2048) to be accessed every 32 msec. A 4K refresh device (12/10) requires 64 msec to refresh all row addresses. DRAM row addresses are refreshed by the Refresh Address Counter, which is triggered by a CAS before RAS refresh cycle.

    The importance, therefore, lies less in whether or not a Macintosh CPU supports 1K, 2K, and/or 4K refresh DRAM parts than whether the Macintosh Memory Controller and DRAM devices have compatible addressing schemes, and whether the DRAM devices are compatible with CAS before RAS refresh cycles every 15.6 use.
  11. How are different sized SIMMs and DIMMs -- 16 Mbytes, 32 Mbytes, etc. -- designed with different types of DRAM devices, such as 4M x 4, 2M x 8, and 1M x 16 parts?

    Consider three different DIMM modules designed using 4M x 4, 2M x 8, and 1M x 16 devices: DIMM #1, DIMM #2, and DIMM #3.

    Remembering that DIMMs and SIMMs have a 64 bit and 32 bit wide data bus, respectively, the following examples can be applied to SIMM modules by using half the number of devices to span 32 bits.

    DIMM #1
    =======
    This is a 32 Mbyte DIMM using 4M x 4 devices in a single bank design. SIMM and DIMM memory modules accommodate one or two memory banks. This DIMM has the following characteristics:
    * a single bank implementation;
    * the memory is laid out to span 64 bits, so 4M x 4 devices require 16 parts per bank;
    * the addressing is common across all DRAM on the module and is multiplexed via RAS and CAS singles, as noted by Control;
    * the module has a density of 4M x 64 (address space of 4M by 64 bits wide), or 32 Mbytes.

    Note: Macintosh computers will size and determine the addressing mode (e.g., 4M x 4, 2M x 8, etc.) of memory modules at system startup.

    DIMM #2
    =======
    This is a 16 Mbyte DIMM using 2M x 8 devices in a single bank. You may be asking yourself: if both 4M x 4 and 2M x 8 parts are considered 16 Mbit devices and both examples are single bank designs, why is DIMM #1's design 32 Mbytes and DIMM #2's design 16 Mbytes?

    Since the data path of a 2M x 8 device is 8 bits wide, rather than 4 bits wide -- as for 4M x 4 parts -- it requires half the number of devices to span across 64 bits.

    Note: DIMM #2 uses 8 devices and DIMM #1 uses 16. DIMM #2 thus has a module density of 2M x 64, or 16, Mbytes.

    DIMM #3
    =======
    This is a 16 Mbyte DIMM using 1M x 16 devices in a two bank design. What's important to note for this particular design is the following:
    * There are two separate memory banks;
    * each bank contains four devices - spanning 64 bits
    * both banks have common address and data buses

    In this example, there are two 1M x 64 banks, resulting in a module density of 2M x 64 or 16 Mbytes.
  12. Do Macintosh computers support composite memory modules?

    No. Apple defines a composite memory module as one that loads the data bus with more than one DRAM input load per bank. Both SIMM and DIMM can accommodate a maximum of two memory banks per module.
  13. Can memory modules with different refresh rates or counts be intermixed?

    For first generation Power Macintosh computers that require SIMMs to be installed in pairs, the two SIMMs within a pair need to be identical. Other pairs can have different supported refresh rates.

    DIMMs with different supported refresh rates can be intermixed. But if it's intended to enable memory interleaving, DIMMs with different refresh rates may or may not have DRAM devices with same addressing modes.

    Note: DRAM devices require the same addressing mode to enable interleaving.
  14. What is EDO memory and do Macintosh computers support it?

    EDO (Extended Data Out -- sometimes called hyperpage) memory are DRAM devices that improve access timing by extending its data out timing while allowing the memory controller to address the next column address. Although EDO devices will improve timing efficiency to main memory by approximately 10%, it does not necessarily mean programs will execute 10% faster. Because much of the time the CPU fetches instructions and data from cached memory, for example, L1 cache within the PowerPC microprocessor and or L2 cache on the main logic board.

    EDO DRAM is a superset of conventional (also called Fast Page Mode) DRAM. This means that an EDO DRAM can be used in place of a Fast Page Mode DRAM, although unless the memory controller is designed to utilize the faster EDO timing, the memory performance will be the same as Fast Page Mode.

    There are three categories of Power Macintosh computers based on their level of EDO memory support. In some Power Macintosh computers, you can use EDO memory and get some potential performance boosts. In others, you can use EDO memory even though you will not derive any benefits from doing so. Finally, there are some Power Macintosh computers in which Apple does not recommend using EDO memory because doing so may damage your computer.

    Power Macintosh Computers that Support EDO Memory

    The following Power Macintosh computers fully support EDO memory and may experience increased performance:

    - Power Macintosh 4400 series
    - Power Macintosh 5500 series
    - Power Macintosh 6500 series
    - Macintosh Performa 6400/200 with internal Zip drive

    Of these computers, ONLY Power Macintosh / Performa 6400 computers support both FPM and EDO memory.

    Only Power Macintosh / Performa 6400 configurations with an internal Zip drive may experience increased performance when using EDO memory. Then, all memory installed must be EDO to take advantage of the benefits. If you mix FPM and EDO RAM, the EDO modules will perform as if they were FPM (just like all 6400 configurations will behave despite the type of RAM installed).

    For additional information see TIL article 19456: PowerMacintosh: Using FPM, EDO, SDRAM, & SGRAM

    Important: There are two types of EDO memory--5 Volt and 3.3 Volt. The Power Macintosh 4400 series requires 3.3 Volt EDO memory whereas the Power Macintosh 5500, 6500, and 6400 computers require 5 Volt EDO. The two types of EDO memory are NOT interchangeable. Most 5 V and 3.3 V EDO memory are physically keyed differently to prevent you from using the wrong type, but even if the DIMM can be installed, verify that you are installing the correct type.

    Additionally, EDO memory can also be used as video memory in the Power Macintosh 4400 ONLY. However, when using EDO memory as video memory, the Power Macintosh 4400 uses 5 Volt EDO memory rather than the 3.3 Volt used as DRAM.

    Power Macintosh Computers that Can Use EDO memory

    Although you will not derive any performance benefits by using EDO DIMMs, you can use them in the following Power Macintosh computers:

    - Power Macintosh 6100 series
    - Power Macintosh 7100 series
    - Power Macintosh 8100 series
    - Power Macintosh 5200 series
    - Power Macintosh 5300 series
    - Power Macintosh 5400 series
    - Power Macintosh 6200 series
    - Power Macintosh 6300 series
    - Power Macintosh 6400 series
    - Power Macintosh 7500 series
    - Power Macintosh 7600 series
    - Power Macintosh 8500 series
    - Power Macintosh 9500 series
    - Macintosh Performa 6360 series
    - Some Macintosh Performa 6400 series


    Power Macintosh Computers that CANNOT use EDO memory

    You cannot use EDO DIMMs in the Power Macintosh 7200 computer. Using EDO memory in the Power Macintosh 7200 computer can cause damage to the logic board and to the DIMMs. Because of this, Apple does not support using EDO memory in the Power Macintosh 7200 computer. Any damage incurred from using EDO memory in the Power Macintosh 7200 computer may not be covered under Apple Computer's limited hardware warranty.
  15. What is parity memory and do Power Macintosh computers support it?

    If a computer is designed for parity RAM, the parity bit allows the memory controller to check for DRAM errors in the form of a parity error. A parity error cannot correct system errors, but the system can be designed to alert the user that a memory error has occurred.

    Apple's newly-introduced Network Server 500 and 700 products are the only computers that support parity memory; all other Power Macintosh Desktop, Tower, and PowerBook computers do not. Parity memory modules add one parity bit to each byte. Thus, a 64 bit DIMM -- eight bytes -- expands from 64 bits to 72 bits wide.
  16. What is ECC memory and do Macintosh computers support it?

    Macintosh computers do not support ECC memory. ECC (Error Correction Coding) memory has additional check bits for each byte. This entails the memory controller calculating and writing error control check bits on each memory write operation, and recalculating and comparing the check bits on memory read operations, and then if necessary correcting the bad bits. As opposed to parity, this can automatically correct single bit RAM errors.
  17. My manual suggests that when upgrading my cache, I use Apple-branded cache. I would like to purchase a 512k cache module, but it does not appear that Apple sells this size. What size should I purchase?

    At this time, 256k is the largest cache module Apple sells for the 7200/7300/7500/7600/8500/8600 models.