Commodore AA+ Chipset

This was the last classic Amiga compatible chipset that Commodore announced in 1992. They planned to release it in 1994 for low end Amiga computers along with AAA.

History

In 1991 Commodore realized that AAA cost was going high, so they postponed it until 1994 and hesitantly designed AGA and released it in 1992 to keep up with the competitors.

Commodore was convinced that even in 1994 AAA systems with their 4 custom chips (6 chips in the 64-bit systems) would be very expensive to use in a low price A1200 like computer or CD32 like console. So unlike Commodore's habit of designing one custom chip for both high end and low end computers to save development costs, Commodore decided to design two custom chips: AAA for the high end computers and AA+ for the low end ones.

In January 1993 at Devcon in Orlando, Florida, Lew Eggebrecht Commodore VP of Engineering at the time stated the following:

"AA+ will be a more profitable version of AA with all the things we wished we'd got in but didn't have time. We have a list of all the problems we currently have at the low end. The serial port, we can't read high density floppies, there isn't enough band width to do 72 Hz screens plus there are no chunky pixel modes for rendering. We listed all those and said, "OK let's go out and fix them as quickly as we can", so AA+ is an extension, not radically new architecture. We're doing the best that we can, taking advantage of advances in technology, significantly reducing the cost and that's the goal."

According to Dave Haynie AA+ only existed on papers and the actual design never started due to Commodore's lack of money at the time. Like AAA and Hombre, Commodore was planning to use AA+ with the Acutiator system that Dave Haynie designed.

A few years later Access Innovations adopted the AA+ name for its BoXeR AGA compatible chipset.

Compatibility

Unlike AAA which was a radical design from ECS and did not support AGA registers, AA+ was built on the foundations of AGA and would be 100% AGA compatible.

Operating System

AA+ systems would be shipped with forthcoming Commodore AmigaOS 4 which added RTG support for chunky pixels.

Chips

To keep costs down, Amiga custom chips would be reduced from 3 (OCS,ECS,AGA) to only two. AA+ would feature two custom chips with 160 - 280 pin packages and each chip would have 100,000 transistors on it. In comparison, AGA Alice has 80,000 while ECS has a total of 60,000. On the other hand, AAA, with its 4 chips, would have a total count of 750,000 transistors, and more than 1,000,000 in its 6 chips 64-bit configuration.

CPU

Commodore stated that AA+ was designed to support ALL 32-bit 680x0 CPUs. For Chunky support, low end systems (A1400) would most likely feature a 68020 with full 32-bit memory addressing (i.e. not 68EC020) or even 68EC030 which could handle RTG drivers easily. Commodore did not add chunky pixels to AGA because RTG which was to be part of Commodore forthcoming AmigaOS 4 required at least 68020 (not 68020EC in A1200) with 4 MB memory at least, while the standard 599 US$ A1200 only had 2 MB and 68020EC CPU.

Memory

AA+ had 8x memory bandwidth over ECS by using 128-bit long memory bus bursts like AAA. Maximum Chip RAM would increase from AGA 2 MB to 8 MB.

AA+ Systems would use 60 ns DRAM, to add Chunky, AA+ systems would need at least 4 MB as a standard to support RTG, most likely A1200 like systems (A1400?) would be shipped with 8 MB which was the standard in 1994 for low end PCs.

Graphics

With 57 MHz pixel clock, AA+ could display progressive rock steady 800 x 600 at 72 Hz resolution in 256 colors easily, or even interlaced 1024 x 768 screens. Perhaps the most significant advancement was the addition of 8- and 16-bit Chunky mode with 24-bit Hybrid mode (3 planes of 8-bit Chunks),although the max resolution for 16-bit pixels would be 640 x 480.

Blitter

A 2x blitter performance over AGA/ECS was promised; compared to AGA 32-bit blitter, AAA blitter had 8x gain in performance because the blitter improved from 16-bit to 32-bit and could use 128-bits long bursts which the AGA blitter couldn't.

The 2x performance increase could be gained by updating AGA blitter from 16-bit to 32-bit without using AAA blitter long bursts which will make cost higher, Commodore never declared whether AA+ would include a 32-bit blitter or utilize 128-bit long bursts like AAA did.

Possibly, the AA+ blitter would stay 16 bit like the AGA one to keep the price down and stay compatible with AGA registers without emulating it like the costly AAA did. Of course Commodore had to add workaround functions to the blitter to improve its speed in 16 bit mode.

A 16-bit performance increase may have also come as a result of using Chunky pixels which the new blitter could handle, AAA blitter was improved to be pixel addressing rather than plane addressing with masks, modulos, and shifts. A new Clip-Rect Line Drawing function was added to AAA blitter for better GUI performance. It is most likely that Commodore would try to copycat some of these new bitter functions to AA+.

By adding Cookie-cut technology to 16-bit AA+ blitter like the Natami team, Commodore made blitting faster by a factor of 2x in comparison to similar planar modes without being forced to use masks like planar modes, just like using hardware sprites in Amiga.

Sound

When asked, Lew Eggebrecht VP of Engineering at Commodore stated that AA+ will support 16-bit sound samples, but it is unclear whether this support would be added by adding a DSP] chip, or by improving Paula to something better like AAA, although Lew Eggebrecht stated once that DSP will be integrated in future low end and high end computers for CD-ROM sound support.

More than 4 channels support like AAA was unconfirmed, sample rates remains unknown but most likely it would be synchronized to AGA Paula to keep it compatible with cycle exact software.

Floppy

AA+ would fully support 1.76 MB HD floppy drives at full speed without using workaround kludges like former Amigas, former HD floppy drives were essentially made to spin at half the speed of standard HD floppy drives to cope with Paula's lack of support of higher bit rates.

Serial Port

AA+ would have two four-byte buffered FIFO serial UARTs like the AAA.

Specifications

• Two Chips with 100k Transistor each.
• Synchronous to video clock.
• 160 - 280 pin packages.
• 32 bit DRAM 60 ns Page Mode Chip Memory.
• 57 MHz pixel clock.
• ECS & AGA registers compatible.
• 4 MB 4 Mbit/s Floppy Controller with Hardware CRC floppy drives using standard technology.
• Support for ALL 32 bit 680x0 CPUs.
• 8x memory bandwidth increase over ECS.
• 2x Blitter Performance (gets twice as many clocks as on AGA).
• Rock steady 800 x 600 x 8-bit Non-Interlace 72 Hz refresh rate, Larger screens at lower refresh rates.
• 16-bit True Color mode (although recent developments with the completion of the first cycle of chipset design indicate that this will actually be a 24-bit True Color Mode)
• FIFO serial ports with large buffer.
• Increased chip ram limit up to 8 MB. With 23-bit memory addressing registers instead of AGA's 21-bit registers.

References

• Lew Eggebrecht at World Of Commodore 92,Toronto reveal information about AA+ and AAA.
• interview with Lew Eggebrecht
• future Commodore machines

See also

• Original Amiga chipset (OCS)
• Enhanced Chip Set (ECS)
• Amiga Advanced Graphics Architecture (AGA)
• AAA chipset (AAA)
• Amiga Ranger Chipset
• Hombre chipset
• List of home computers by video hardware

External links

• ACUTIATOR
• The Dave Haynie Archive with lots of detailed info & specs