Chip RAM and Fast RAM

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Chip RAM and Fast RAM are certain RAM regions in Amiga - computer system of Commodore . The Amiga's chipset can only access the chip RAM to fetch video and sound data from it, among other things. For details see the original chip set .

Chip RAM

Amiga hardware structure with the special chipset, Chip-RAM and Fast-RAM

In addition to the Amiga chipset, the main processor of the Amiga can also access the chip RAM . Normal programs or data can therefore also be saved and processed there. Processor bus and chipset bus are synchronized and interconnected for access. Therefore, the processor must wait when accessing the chipset, while when the processor accesses other memory areas, both bus systems are separated, so these accesses can take place unchecked by the chipset (see below). This is why these other memory areas are also called Fast RAM.

The mixing function of the chip RAM for program and video data is similar to the UMA principle in the PC area.

In terms of hardware , the chip RAM is designed with dynamic RAM . The necessary refresh cycles - in contrast to those of the ( real ) Fast RAM - are also carried out by the chipset. In many Amiga models, the chip RAM is firmly soldered onto the Amiga motherboard, so it is not plugged in there. (Exceptions: the Amiga 1000 has 256 KB built-in and a further 256 KB can be plugged into the front flap as a memory expansion; the Amiga 500 adds 512 KB via a plug-in card; the Amiga 3000 has 1 MB and the Amiga 4000 has a 72-pin SIMM executed.)

The chip RAM is always at the very beginning of the address range, i.e. from memory address $ 00000000. At the time of booting the 68xxx processor must have the boot ROM there , which is therefore displayed ("mirrored") there by the reset . However, there is an immediate jump to the address actually intended for the ROM, where the mirroring of the ROM is deactivated from address $ 00000000 during the initialization of the system.

The size of the addressable chip RAM is determined by the Agnus chip of the Amiga chipset:

  • with the OCS up to 512 KB;
  • with the ECS up to 1 or 2 MB (with Fat Agnus );
  • with AGA / AA up to 2 MB (with Alice instead of Agnus).

Most of the Amiga models (A500 (+), A600, A1200, CDTV, CD32) were factory-equipped exclusively with chip RAM by Commodore.

The timing of the chip RAM is completely determined by the video functionality and its TV compatibility. This means that there are significant differences in the data for the PAL and NTSC versions of the chips and the Amiga models equipped with them. In particular, the Agnus chip with its DMA access to various types of data is set up on a fixed grid of how many bytes per display line must be fetched from the chip RAM at what point in time and for what purpose. As a result, the speed cannot simply be increased by increasing the clock rate of the main processor as long as it is accessing the chip RAM; then the video timing would no longer be correct. Accelerations could only be achieved by completely redesigning the chipset , which would jeopardize backward compatibility . Accordingly, this step did not take place in Amiga history at the time of Commodore. Over time, the chip RAM became a serious performance bottleneck for the entire Amiga system. The age of plug-in graphics cards began as early as the Commodore era . They did not have such restrictions and were able to generate significantly more resolutions and color depths.

Fast RAM

In addition to the chip RAM, there can also be Fast RAM in the Amiga system , which can only be accessed by the main processor, but not by the chipset. Here the processor can play its speed unchecked. The AmigaOS operating system therefore preferably places programs in this area if Fast-RAM is available. Chip-RAM is only used as program and data memory when the Fast-RAM is exhausted. This leaves the (small) chip RAM largely free for video and sound data.

There are several different types of Fast RAM (with decreasing speed):

  • 32 bit:
    • It is arranged on a processor plug-in card ( accelerator card ) (if available) and is directly and optimally connected to the local bus of this processor.
    • It is located in sockets on the main board of the computer (only on the larger models of the A3000 and A4000 series). The Ramsey RAM controller attaches the memory at the speed of the original processor.
    • With the Amiga 1200 , up to 8 MB Fast-RAM can be added via an expansion card in the lower slot.
    • It is connected via a 32-bit Zorro III expansion card (A3000 / T, A4000 / T), which can be accessed by a 32-bit CPU with full bus width but possibly only at reduced speed.
  • 16 bit:
    • With the first Amiga 2000 A, up to 1 MB Fast-RAM is connected via a card in the processor slot.
    • It is integrated via a 16-bit Zorro II expansion card (A2000, also A3000, A4000) (max. 8 MB). RAM on Zorro II cards can only be accessed by processors with a 32-bit data bus in the A3000 and A4000 with 16-bit access, which means that only low speeds are achieved. A Zorro III card that also allows 32-bit access makes more sense.
    • With the Amiga 600 or Amiga 1200 it is arranged on a PCMCIA card. This interface is only 16 bits wide, so that such RAM does not reach full speed with the A1200.

In terms of hardware, the Fast RAM has very different memory addresses, depending on the model, type of expansion (see above) and processor size. On the Zorro bus, its address location is even configured dynamically at the time of boot, so that several RAM cards can also be plugged into Zorro slots in parallel. The operating system is flexible enough to correctly take into account all of these possible configurations.

"Ranger Memory"

In addition to Fast-RAM and Chip-RAM, there is also a third type of memory - sometimes called Ranger Memory , fake Fast-RAM or Slow RAM . It is located on memory expansion cards for the internal expansion space in the base of the A500 or on the mainboard of the A2000. Like the chip RAM, this memory is controlled and refreshed via the chipset, but cannot be addressed by the chipset itself (the registers of the original Agnus have not been expanded). As a memory expansion that is inexpensive to manufacture - it practically only contains the memory modules and does not require any additional components such as a Zorro memory expansion - it combines the disadvantages of Fast RAM and Chip RAM - there is no expansion of the chipset memory for video and sound Data takes place, nevertheless the memory accesses of the processor like those on chip RAM are slowed down by the chipset. However, the speed difference between chip and real Fast RAM in the A500 with its standard 68000 clocked at almost eight megahertz is noticeable, but usually not very large.

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