MOS Technology Agnus
The Agnus (Latin for lamb) is one of the custom chips in the chip of the Commodore Amiga. In the later versions it was renamed Alice.
" Agnus " is an abbreviation of "Address Generator" (English Address Generator Units).
The module is responsible for the control of the so-called chip RAM in the Amiga, the video sync signals (HSY, CSY, BBY ) and contains the coprocessors Copper and blitter. Agnus also contains the complete DMA logic for all six possible sources. For the timing of the different DMA accesses a screen line is used as a time reference. In each line, 225 memory accesses by Agnus on the DMA channels and the CPU are distributed. For the correct timing a row and a column counter charge (HSY, BBY ). On these signals, the timing can also be controlled externally ( genlock ).
An essential main component of the Agnus large address generator. All custom chips of the Amiga can access independently from the CPU to the RAM. This, however, these chips have an available DMA channel, and the address generator. Via the DMA channels, the access to the RAM is settled. The address generator converts the signals of a chip in complete memory addresses to specific memory addresses or assigns individual registers in the chips. It serves almost as a kind of translator, which determines the required address in the memory. One can imagine something like that from a specific location instead of the postal address only the description " first road to the entrance and then third house on the left " is known. The address generator makes such information mutatis mutandis " Schiller Straße 12, 47324 block of living".
None of the other chips of the Amiga chipset has DMA channels or address generators. Therefore, all memory accesses are managed centrally via Agnus and controlled by him and co-ordinated. Overall Agnus has 25 DMA channels, write about the 25 different devices without the aid of the CPU data into memory or read from it. Retrieved from these from the Agnus chip RAM and illustrated by Denise video data includes in addition to the pixel data and palette data and the sprite Copper lists. The data of BOBs are indeed brought by Agnus by DMA, but then copied using the integrated in Agnus blitter be displayed directly to the position within the video memory in the chip RAM.
Agnus brings to a video and sound data from the chip - RAM and can recopy on the other hand data within the chip RAMs and this also process ( blit ), but can also generate data, namely the drawing of lines and surface filling with the blitter.
The original version of the Agnus can 512K chip RAM address (OCS ). It was made in the DIP package and only used in the Amiga 1000 and Amiga 2000 with A- Board. The successor models Amiga 500 and Amiga 2000 with B- Board already have a PLCC socket for the basis of this housing form so-called Fat Agnus (see below for PLCC versions). The Fat Agnus appeared in different versions for different chip RAM sizes with differing functions (OCS / ECS ). Additionally for the ECS versions of the Fat Agnus circulating non-embossed by the manufacturer and therefore inconsistent terms used such as Big Agnus, Super Agnus or a combination of these.
The Alice - chip is the evolution of Fat Agnus for the AGA ( AA) chipset. Alice was tailored newly developed on the basis of the A3000 Agnus and especially on the newer CPUs (from MC68020 ). Because these processors, the bus is 32 bits wide, some of the signals could be combined or removed. This allowed 2 MB Chip RAM can be used. However, Alice can not be installed in old Amigas. It is only for the Amiga with AGA ( AA) Chipset been developed and is in the Amiga 1200, Amiga 4000 models and used in the CD ³ ².
- 8361 ( A1000, A2000A, OCS, DIP package, NTSC )
- 8367 ( A1000, A2000A, OCS, DIP package, PAL)
- 8370 ( A500, A2000, OCS, PLCC package, NTSC )
- 8371 ( A500, A2000, OCS, PLCC package, PAL)
- 8372 ( A500, A2000, ECS, 1 MB Chip RAM )
- 8372 -A ( A500, A2000, ECS, 1 MB Chip RAM )
- 8372 -B ( A500 , A600, ECS, 2 MB Chip RAM )
- 8372 -AB ( A3000 (T), ECS, 2 MB Chip RAM )
- 8374 Alice ( A1200, A4000 (T ), CD ³ ², AGA)
- 8375 ( A500 , A600, ECS, 2 MB Chip RAM )
The Fat - ( PLCC ) - Agnus 8370-72/75 therefore exist in different versions:
ECS chips are switchable between PAL and NTSC, exist in the OCS dedicated versions.
When replacing / rebuilding the pinout is interesting. The type information is intended only for reference; of a present Agnus chip can not be closed directly to the pin assignment, as this seems to have been determined only at the Packaging regardless of the chip.