Atari 130XE User Manual download pdf (Page 56)

8 For experts: Technical details

“Ultimate 1MB BIOS”. It neither contains a standard interrupt handling nor a character

set at $E000. Therefore the screen is completely garbled when the freezer is activated

from within the “Ultimate 1MB BIOS”.

8.4 512k RAM extension

A separate 512k RAM chip is included for the 512k RAM extension. This way the freezer,

the cartridge emulation with modules in the freezer RAM, and the RAM extension can be

used in parallel to the full extent.

Because the PIA signals are not available on the PBI, the CPLD must mimic the

behavior of the PIA registers. The PIA in the Atari keeps working “in parallel”, so BASIC,

OS ROM, and Self Test are controlled as usual. Read access to the PIA is ignored by the

CPLD,

i. e.

in this case the Atari reads the real PIA registers as usual. The logic only

reacts on write access to the addresses $D301 (

PORTB

) and $D303 (

PBCTL

). The CPLD

also saves the value of bit 2 of

PBCTL

, which controls if the access is performed on the data

The CPLD stores the data of a write access to $D301 either in the internal data register

or in the internal data direction register. If a PIA pin it setup as an input, then pull-up

resistors in the Atari pull it “high”. If a PIA pin is setup as an output, then the data

The CPLD emulates exactly this behavior and therefore behaves exactly like every other

RAM extension in the Atari. The Oldrunner OS for example sets

PORTB

up as input

because the 400/800 had 4 joystick ports – but no RAM/ROM control via

PORTB

. This

automatically deactivates every RAM extension, because bit 4 of

PORTB

must be “low” to

activate the RAM extension. But the emulated pull-up resistor in the CPLD pulls the pin

“high” automatically in this case, like it would have been the case in the PIA.

The rest of the RAM extension logic is relatively simple. If the “

Ramdisk

” switch is

turned

and if bit 4 of the emulated

PORTB

, then the internal

memory of the Atari is disabled for every access to $4000. . . $7FFF and a 16k bank of

the 512k RAM extension is mirrored in instead. The bits 2,3,5,6,7 of the emulated

PORTB

8.5 Oldrunner

The Oldrunner is active if the “

OldOS

” switch is turned to

). For every access to

$E000. . . $FFFF bank 127 of the ﬂash ROM is mirrored in and for every access to

$C000. . . $CFFF the internal memory is deactivated. This makes the Atari behave exactly

like the original Atari 400/800, which did not have any memory at $C000. . . $CFFF. When

the Math Pack area ($D800. . . $DFFF) is accessed, the internal OS ROM of the Atari will

be mirrored in and the Math Pack contained in the OS will be used.

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Atari 130XE User Manual Page 56

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