Let me give Tamas Bene of HARD the keyboard...
"...It depends on what we call "standard".
My expansion is 130XE-compatible (without Antic-switching), so all programs
which is written for XE should run as well on my Atari.
Actually it is a 192K expansion, so the whole memory is 256K.
The problem is that demo-coders (and others) use $d301 in a non-standard way.
I mean they change bits in $d301 which work differently on different
expansions, or even they store values in $d301 other than $fc-$ff, instead
of masking only those bits which are intended to be changed.
(Fortunately the Atari OS was written much more carefully, they even use
bit masking for setting values like $ff. Otherwise even the OS might not
work with extended machines. :^)
For example, I found the following instructions in the Journey demo
at several places:
lda #$b3
sta $d301
This has some meaning on a particular expansion, but I have no idea what
it should do. For me, it just switches OS in and BASIC out.
BTW, my 192K expansion works as follows:
bits 0,1 and 7 work as for normal Atari (switching OS/Basic/Selftest).
bit 4 switches the expansion on/off, just like for 130XE.
here is a table for bits 2,3,5,6:
bit6 bit5 bit3 bit2 action ------------------------------ 0 0 0 0 maps base memory $0000-$3fff to $4000-$7fff 0 0 0 1 maps base memory $4000-$7fff to $4000-$7fff 0 0 1 0 maps base memory $8000-$bfff to $4000-$7fff 0 0 1 1 maps base memory $c000-$ffff to $4000-$7fff 0 1 0 0 extended bank #01 at $4000-$7fff 0 1 0 1 extended bank #02 at $4000-$7fff 0 1 1 0 extended bank #03 at $4000-$7fff 0 1 1 1 extended bank #04 at $4000-$7fff 1 0 0 0 extended bank #05 at $4000-$7fff 1 0 0 1 extended bank #06 at $4000-$7fff 1 0 1 0 extended bank #07 at $4000-$7fff 1 0 1 1 extended bank #08 at $4000-$7fff 1 1 0 0 extended bank #09 at $4000-$7fff 1 1 0 1 extended bank #10 at $4000-$7fff 1 1 1 0 extended bank #11 at $4000-$7fff 1 1 1 1 extended bank #12 at $4000-$7fff
This expansion is quite a standard, it was published in the german Atari Magazin a couple of years ago. Also, it needs the simplest hardware among all the extensions I know of, so it's the cheapest one too. I'm sure a lot of people built it for this reason.
Most programs using extended memory do some sort of memory check (at least the self-respecting ones ;-) But they usually fail on my expansion because they detect expanded banks in the first 4 cases (bit5=bit6=0) even though in these cases the base memory is mapped to the bankswitched $4000-$7fff area. I wonder why they don't do a check for this case. It would be simple - just treat the 4 16K "banks" of the base memory in the same way as expanded banks, and check if writing to any bank causes any changes in any other bank. The check could look like this:
______________________________________ /__ __ __ _\ / / /_|_ | |__\ Software, Hungary ______________________________________
Here is another Description by Dracon...
XL/XE RAM expansions
There are some types of RAM expansions designed for XL and XE
computers. The most popular in Europe are based on the general
concept used by the Atari Corp. in the 130XE computer: a port,
formerly responsible for controlling joysticks #2 and #3, is now used
as a memory controller port. This port is mapped at $D301 and
traditionally labelled as PORTB.
The general purpose of the PORTB is to control all on-board memory
resources, i.e. the system RAM as well as the OS and BASIC ROMs.
I. ROM controller
Bits #0, #1 and #7 control the system ROM. This function is common
for all XL and XE computers:
Bit Meaning (if set)
--- ----------------
#0 OS ROM enable
#1 Atari BASIC ROM disable
..
#7 SELF TEST ROM disable
Bit #0 changes affect the XL OS area at $C000-$CFFF and $D800-$FFFF,
if the bit is set, there is the XL OS ROM there, otherwise the shadow
RAM is enabled. NOTE: the PDVREG $D1FF has higher priority than PORTB!
Bit #1 affects the area $A000-$BFFF. If this bit is cleared, there's
Atari BASIC ROM there, RAM otherwise.
Bit #7 affects the area $5000-$57FF. If it is cleared, the SELF TEST
ROM, located normally at $D000-$D7FF and masked out by hardware I/O
registers, is switched to $5000-$57FF; otherwise, there's normal RAM
there.
II. RAM controller
1) Atari 130XE
Stock 800XL and 65XE computers lack additional RAM, so the remaining
PORTB bits do nothing. 130XE however and expanded XL/XE use them to
control RAM extensions.
Generally, considering the 16-bit address bus, it is impossible to
enlarge the physical address space past the existing 64k. In the
130XE, the total amount of system RAM is split into two large parts:
64k base RAM and 64k bank select RAM. That last is accessible as four
16k portions to be exchanged with a 16k block belonging to the base
RAM area.
This area is located from $4000 up to $7FFF. Both 6502 and Antic are
granted an *independent* access to this memory. PORTB bits meaning is
as follows:
bits #2, #3 - bank select
bits #4, #5 - access control
bit #6 - unused
The first pair (bits 2 & 3) selects one of four additional 16k banks
you want to access to. The other pair decides, which processor will
use the bank selected by the first two bits:
#4 - CPU access: 0 - additional RAM
1 - base RAM
#5 - Antic access: 0 - additional RAM
1 - base RAM
This gives four possible combinations:
76543210
--------
xx00xxxx - both processors use the additional bank
xx01xxxx - only Antic uses the additional RAM
xx10xxxx - only CPU uses the additional bank
xx11xxxx - both processors use the base RAM
That's all for 130XE.
2) 192k RAM expansion
The computer's behaviour is exactly the same as in 130XE besides of
that bit #6 is used together with the bits #2 and #3 to select an
additional banks. This gives eight combinations, i.e. eight banks may
be accessed, 16k each.
3) 320k Compy Shop expansion
Also known as "German 320k". This type of RAM extension is quite
popular in Europe, so that several American programs get confused
considering this to be 192k.
Since all free bits have been used for bank selection, the Compy Shop
expansion uses bit #7 for bank selection. This gives 16 possible
combinations, hence the additional RAM area can be enlarged to 256k
(256 + 64 = 320).
To get rid of possible conflicts with the SELF TEST ROM (normally
controlled by the bit #7), this bit is used to control SELF TEST only
when bits #4 & #5 are both set to 1. Otherwise it controls the bank
select RAM.
4) 256k Newell Industries expansion
This expansion is NOT fully 130XE compatible. It is similar to the
192k with except that the bit #5 is used for bank selection, not for
Antic access control. It means, that you have four bits for bank
selection, but both CPU and Antic must use the same memory at a time.
Furthermore, only 12 of 16 possible combinations are used so that you
have 192k additional RAM instead of 256k. This is the most stupid
expansion I have ever seen (my computer has it installed).
4) 320k Newell Industries expansion
Also known as "American 320k". This expansion is NOT fully compatible
with the 130XE standard, but it was apparently quite popular in the
USA since some software (f.e. ICD's SpartaDOS X 4.x) seems to base on
it. Nevertheless, some software, like "Envision" for example, may
fail on such computers.
Only bit #4 is used to control the memory access here, which implies
that there's no separated access for the CPU and Antic. In fact, both
processors must use the same type of RAM at the same time; if bit #4
is set, CPU and Antic use the base RAM, otherwise both use additional
bank.
Bits #2, #3, #5 and #6 are used for bank selection.
5) 576k expansion.
This is similar to the American 320k with except that bit #7 is used
the same way as in Compy Shop 320k expansion. So bits #2, #3, #5, #6
and #7 are used for bank selection if the bit #4 is set to zero
(CPU & Antic additional RAM access). This expansion is not fully
compatible with the 130XE.
6) 1 MB (1088k) expansion.
The same as 576k with except that the bit #1 (normally controlling
Atari BASIC ROM) is used for bank selection if bit #4 is set to zero.
This gives six bits (64 banks, 16k each) for bank selection, so that
the additional bank area may be enlarged to 1024k. This type of
expansion isn't fully 130XE compatible either.
III. RAM detection routine
Since it is quite difficult to determine what expansion a particular
computer has and what amount of memory is currently available, the
most simple solution is to use the following routine. It is a
non-destructive test, any data previously stored in the additional
RAM will remain intact.
The routine returns the number of found additional RAM in the
accumulator. You must multiply it by 16 then add 64 to get the total
amount of the system RAM in kilobytes as a result.
; Bank select RAM detection
; Stolen from the SysInfo
;
; Must NOT be located within the $4000-$7FFF !!!
;
portb = $d301
extra = $4000
;
lda portb
pha
lda #$00
sta counter
ldx #$7f ;Save the ramdisk contents before the test
loop1 txa
asl
ora #$01
sta portb
lda extra
sta buffer,x
dex
bpl loop1
ldx #$7f ;Clear tested RAM.
loop2 txa
asl
ora #$01
sta portb
lda #$00
sta extra
dex
bpl loop2
lda #$ff ;Reset PORTB
sta portb
lda #'K ;Mark base RAM as tested.
sta extra
ldx #$7f ;Count banks.
loop3 txa
asl
ora #$01
sta portb
lda extra
bne skip
inc counter
lda #'K
sta extra
skip dex
bpl loop3
ldx #$7f ;Restore the ramdisk contents
loop4 txa
asl
ora #$01
sta portb
lda buffer,x
sta extra
dex
bpl loop4
pla
sta portb
lda counter
rts
;
counter .dc 0 ;number of banks
buffer .ds 64 ;64-byte buffer
Konrad M.Kokoszkiewicz
mail:draco@mi.com.pl
http://www.orient.uw.edu.pl/~conradus/
IRC:[Draco]
*** Ea natura multitudinis est,
*** aut servit humiliter, aut superbe dominatur.
*************************************************
*** U pospolstwa normalne jest, ze albo sluzy ono
*** unizenie, albo bezczelnie sie panoszy.
(Liv. XXIV, 25)
Last changes: 19 Jun 1997
Feel free to contact me for any legal reason!
HeAvEn, Member of TaquarT