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2. Two kinds of RAM in the TT
==========================
The TT sports two kinds of RAM - So called "dual purpose RAM",
commonly known as ST-RAM, and "single purpose RAM", also known as
TT-RAM. The ST-RAM is, as seen by the programmer, identical to the
RAM in the ST and is being shared by CPU and all the custom-chips.
It appears in the RAM-space of the 68030 at adress
$00000008 to $009FFFFF
The most significant difference to the RAM in the ST is the way it
is internally organised since it allows the TT-Shifter to access
64 bits at once. This, however, is of no concern for the
programmer.
The ST-RAM is, in theory, expandable from $00A00000 to $00DFFFFF.
The other kind of RAM maps the remaining adress space of the
MC68030. It (commonly) supports burst-mode access for lightning
fast fills of the 68030-cache and cannot be accessed by any other
device of the TT. Hence, it only supports 32-bit wide accesses,
which is however sufficient for the CPU.
For the programmer:
===================
For sensible support of multitasking or advanced systems, a
program should always "start" with an MSHRINK GEMDOS-command to
"free" the RAM that the program does not require. TOS memory
management is somewhat simple, so each program is being given the
whole free RAM of the computer and it is being relied on that the
program returns the unused RAM.
If the program requires additional RAM, this should be done via
either a MALLOC, or better yet, an MXALLOC GEMDOS-command.
Please refer to the GEMDOS documentation for the usage of MSHRINK
to reduce the memory usage of your program.
The important difference between MALLOC and MXALLOC is that MALLOC
will always allocate the RAM-type specified in the file-flags.
However, if file-flags are set to allocate TT-RAM via MALLOC and
this certain program uses MALLOC to request screen memory, the
program will not work since screen memory has to be in ST-RAM.
MXALLOC allows to specify which RAM to allocate within the command
itself. The usage is as follows:
move.w mode,-(sp) ;which RAM to allocate
move.l amount,-(sp) ;number of byte to allocate
move.w #$44,-(sp) ;GEMDOS function number
trap #1 ;execute GEMDOS
addq.l #8,sp ;clean up stack
amount contains the number of bytes to request, mode allows the
following settings:
mode = 0 allocate ST-RAM only, error if not enough ST-RAM free
mode = 1 allocate TT-RAM only, error if not enough TT-RAM free
mode = 2 allocate what is free, preference on ST-RAM
mode = 3 allocate what is free, preference on TT-RAM
Using MXALLOC allows the programmer to both allocate ST- and
TT-RAM specifically for all purposes.
Please note that ALL memory-areas that customchips need to access,
like video memory (Shifter), sampled sound data (DMA-sound chip),
I/O-buffers (DMA in general) need to be in ST-RAM.
It is unwise to use MALLOC and restrict the program to use ST-RAM
via the fileflags, even if the program is prefered to run on an ST
and only meant to be TT-compatible. TT-RAM is lightning fast in
comparison to ST-RAM and will result in a tremendous speedup in
all CPU-intense tasks.
Compatibilities and incompatibilities
=====================================
=> In comparison to the Atari ST and Atari STE:
This computer series does not feature TT-RAM in general. There
are some expansions that have TT-RAM, but the number is too small
to list them in detail here. An MXALLOC-call that requires TT-RAM
will fail to work on these machines, but an MXALLOC-call that
just prefers TT-RAM will allocate ST-RAM (if free).
=> In comparison to the Atari Falcon030:
The Falcon030 also has, in general, no TT-RAM. Certain
accelerator cards (FX card, Centurbo 2, etc.) do have TT-RAM
though, some even allow burst-mode access, just like the TT does.
In these cases, it's also sensible to write TT-RAM compatible
software by using MXALLOC that prefers but does not require
TT-RAM.
=> What to look for when writing ST-Programs:
Already dealt with in the chapter "for the programmer".
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