ScuzzBlog: Diaries September 2020

Entry 11th September 2020: Post 1: Amiga AVAIL - If memory serves me.

Amiga AVAIL - If memory serves me.

From memory if I recall the command AVAIL [ short for available ]
will report the available CHIP and FAST memory.

OK... digs a little deeper into the old brain bank and me thinks
CHIP is up to 2MB on an Amiga and can be accessed by all the
custom chips etc. The FAST or other RAM as provided by cards is
not accessible by the custom chips and is reserved for use by
the CPU. But don't quote me on it.

AVAIL can be interrogated further with CHIP, FAST, VIRTUAL, TOTAL
and FLUSH.

SHOW gives the option to define the way the figures are reported.
i.e. BYTE, KILO or MEGA.

When using the secondary request the result will show just that
and nothing else. So if FAST is requested it shows just FAST.

AVAIL FLUSH will only remove a FONT or LIBRARY that is in use
without having to reboot the computer.

[ UNRELATED BUT USEFUL ] ENDSHELL ends a process and closes the


The DMA controller is integrated into the AGNUS or ALICE on AGA

The way memory is allocated to the CPU and chipset members is
by way of the AGNUS. This allows custom chips to perform operations
such as audio and video and other DMA operations that are independent
of the CPU.

A shared RAM data bus 16-bit is used on OCS and ECS machines. The
AGA systems use the 32-bit data bus and is controlled by the Alice
coprocessor which replaces the AGNUS. Control is also given to the
32-bit RAM on the AGA machines. Interestingly the bandwidth of
the chipset RAM is increased fourfold compared to the earlier models.

CHIP RAM amount is controlled by the AGNUS/ALICE. The earlier 1000
and 2000 systems could only address 512KB of CHIP RAM via a 48 pin
DIP. This was upgraded to a socketed or surface mounted  84-pin PLCC.
This means that all models other than the 1000 can be upgraded to
2MB of CHIP RAM. The maximum on any Amiga model is 2MB. Interestingly
the A4000 has a redundant jumper that allows for larger CHIP RAM
but is limited to 2MB by the ALICE.

Amigas can be further expanded with the use of FAST RAM though
this is only accessible by the CPU and not the custom chips.
Kinda makes the processes faster using FAST as they are not blocked
by the activities of the custom set.

The earlier versions of the 2000 and the more common trapdoor memory
cards of the Amiga 500 contain 512KB pseudo-FAST RAM or slow RAM
and is controlled by the AGNUS. These have the same limitations as
CHIP RAM without the functionality due to register restrictions.
This SLOW RAM was extended to 1.8MB though required a GARY adaptor.

AND FINALLY, one exception to the rule about video and sound being
stored in FAST RAM is the implementation used by the Vidi Amiga
unit allowing it to play longer videos. It is not technically
visible as it simply transfers the data to CHIP RAM just before
being called upon. Clever or what !!

OK so what have I shown in the pics. First there is an early BLOCK
DIAGRAM showing the way chips generally operated.

There is a definition of the AVAIL function plus a descriptive on
the way a RAM disk works.

Next up we have the AVAIL command in action. Plus the memory
needed by a largish animation which combines CHIP and FAST as a
way of executing the size requirement.

Next up my beloved Amiga 1200 from 1993. She has the customary 2MB
CHIP RAM and 32MB FAST on a Blizzard 1230 IV. The card also holds
an SCSI KIT that permits a number of external SCSI devices and these
are all picked up by the SCSI controller. Note the vast number of
drives available including ZIP and CD-ROM.

CHIP and FAST are always shown at the base of Directory Opus and
also combine the figures to give a total that is very helpful when
playing videos.

Anyway that is all I know. Read more on the subject if you want
some real details on the matter. One last thing and that is not
all RAM is equally fast or even FAST. If you have ever seen the
sound and vision on Sensi Soccer do catchy up then you will know
what I mean. So funny. PCMCIA does have its restrictions though
was never fully refined by the time the A1200 was released. Don't
think you could blame Commodore for that.

Amiga AVAIL - If memory serves me.

Amiga Technolgies A1200 with GVP JAWS
and Amitek 4MB PCMCIA RAM on a washed
out Commodore 1942.

2nd Amiga 1200

Blizzard 1230IV with 32MB RAM.

Amiga 600/1200 - PCMCIA

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Last updated 12th September 2020

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scuzzscink 2020