Amiga Chip RAM

Chip RAM and Fast RAM are certain RAM areas in the Amiga computer system of Commodore. The chipset of the Amiga can only access the chip RAM to fetch it, among others, video and sound data. For details see the original chip set.

Chip RAM

On the chip RAM can next to the Amiga chipset also the main processor of the Amiga to access. There can therefore be stored and processed even normal programs or data. To access processor and Chipset are synchronized and interconnected. Therefore must processor side to wait for the chipset when accessing while on other storage areas both bus systems are disconnected while accessing the processor, these accesses can therefore be unrestrained by the chipset ( see below). Therefore, we call these other storage areas and Fast RAM.

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

In terms of hardware, the chip - RAM is designed with dynamic RAM. The necessary refresh cycles - also performed by the chipset - as opposed to those of the (real) Fast RAM. The chip RAM is soldered on many Amiga models firmly on the Amiga motherboard, there so not pluggable. ( Exceptions: the Amiga 1000 256 KB are installed and another 256 KB inserted as a memory expansion on the front flap, the Amiga 500 are added 512K via a plug-in card, the Amiga 3000 1 MB is socketed and the Amiga 4000, it is a 72 -pin SIMM carried out. )

The chip RAM is always at the very beginning of the address range, ie from memory address $ 00 millionth To boot time must be at 68xxx processor where the boot ROM are, which therefore appears by the reset there ( "mirrored" ) is. However, it takes place immediately jump to the actually intended for the ROM address, then where in the course of the initialization of the system mirroring the ROM is disabled from address $ 00 million.

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

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

Most Amiga models ( A500 ( ), A600, A1200, CDTV, CD32 ) were fitted by Commodore factory exclusively with chip RAM.

The timing of the RAM chip is completely determined by the video function and the television compatibility. So that there are also significant differences in the data for the PAL and NTSC versions of chips and furnished with them Amiga models. In particular, the Agnus chip with its DMA accesses to various kinds of data is set to a fixed grid, the number of bytes to be fetched per display line at which point the purpose for which off-chip RAM. The result is that the speed can not be increased by simply increasing the main processor clock, as long as it accesses the RAM chip; then the video timing would no longer apply. Speed ​​could be achieved only by complete backward compatibility hazardous redesign the chipset. According to this step did not take place in the Amiga history in the days of Commodore. The chip RAM was time to a serious performance bottleneck for the entire Amiga system. Already on Commodore times but also began the era of pluggable graphics cards that have no such restrictions and were able to generate a lot more resolutions and color depths.

Fast RAM

In addition to chip RAM in the Amiga system can also provide fast RAM to which only the main processor can access, but not the chipset. Here, the processor can exploit its speed unchecked. The operating system AmigaOS therefore introduces programs in the presence of fast RAM preferably in this area. Only when the fast RAM is exhausted, use is made of chip RAM as program and data memory. Thus, the ( small ) chip RAM remains largely free video and sound data.

There are several different types of fast RAM ( in descending order of speed):

  • 32-bit: It is located on a processor plug-in card ( accelerator ) ( if any) and connected directly and optimally to the local bus of this processor fast.
  • It plugged into sockets on the motherboard of the computer ( only the larger models, the A3000 and A4000 series). RAM controller Ramsey binds to the memory with the speed of the original processor.
  • When Amiga 1200 up to 8 MB Fast RAM can be added via an expansion card in the lower slot.
  • It is a 32 -bit Zorro III expansion slot card connected ( A3000 / T, A4000 / T), on a 32 -bit CPU with full bus width but can access only possibly reduced speed.
  • Up to 1 MB Fast RAM are connected at the first Amiga 2000A on a card in the processor slot.
  • It is a 16 -bit Zorro II expansion slot card ( A2000, A3000 also, A4000 ) involved ( max. 8 MB). RAM on Zorro II cards can of processors with 32 -bit data bus A4000 achieved only with 16- bit accesses and the A3000, so that only small velocity is reached. It makes sense is a Zorro III card, which allows 32- bit accesses.
  • It is located at 1200 Amiga 600 or Amiga on a PCMCIA card. This interface is only 16 bits wide, so that such RAM at A1200 does not reach full speed.

Hardware default, the Fast RAM is very different memory addresses, depending on the model, extension type (see above) and processor size. At the Zorro bus its address location is even dynamically configured at boot time, so that you can put in Zorro slots also several RAM cards in parallel. The operating system is flexible enough to consider all of these possible configurations correctly.

" Ranger Memory"

In addition to fast RAM and chip RAM, there is a third type of memory - sometimes Ranger Memory, spurious fast RAM or RAM called Slow. It is located on memory expansion cards for the internal expansion space in the bottom of the A500 or on the mainboard of the A2000. This memory is like the RAM chip controlled and replenished at the chipset can from this, however, are not addressed itself ( the registers of the original Agnus were not extended). As inexpensive to manufacture memory expansion - it contains practically only the memory modules and does not require significant additional components such as a Zorro - memory extension - it combines the disadvantages of fast RAM and chip RAM - there is no extension of the chipset memory for video and sound data instead, still being slowed the memory accesses of the processor as the chip- RAM by the chipset. However, the difference in speed between chip and real fast RAM is the A500 with its clocked at just under eight megahertz 68000 standard, although noticeable, but usually not very large.

Commodore International Booting Read-only memory MOS Technology Agnus Amiga Enhanced Chip Set Amiga Advanced Graphics Architecture Amiga Zorro II
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