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8 For experts: Technical details

8.1 Hardware

The heart of the

TURBO FREEZER

is the Xilinx XC95144XL CPLD. It contains the

complete logic of the freezer and controls the freezer RAM as well as the cartridge emulation.

There are 1MB ﬂash ROM and 1MB battery backed freezer RAM present which are shared

between the functional blocks, i.e. freezer, cartridge emulation and 512k RAM extension.

When the freezer RAM or the ﬂash ROM of the freezer have to be mirrored into the

Atari, the internal RAM and ROM of the Atari must be deactivated. This is achieved using

a small trick. The freezer pulls the line of the ANTIC refresh pin, which is intended to be

an output, to “low”. This makes the MMU in the Atari believe that ANTIC is performing

a memory refresh, hence it deactivates all built-in memory. According to Bernhard Engl,

who works as a full-time chip designer nowadays, it is not dangerous to pull a “high” level

“low” from outside in the NMOS-Depletion-Load technology, because that is how NMOS

works anyway. If the ANTIC had been manufactured in CMOS technology, this approach

would have been dangerous and could have damaged the ANTIC – and there never had

been a TURBO FREEZER.

8.2 Memory partitions

8.2.1 Freezer RAM

512k of the freezer RAM are directly assigned to the 512k RAM extension. The remaining

512k RAM are shared between the freezer and the cartridge emulation. The freezer can

access the upper 128k (banks 48. . . 63) of this area. The topmost 16k (banks 62 and 63)

are exclusively reserved for the freezer (

i. e.

for the hardware register shadowing). The

bank switching in the freezer addresses the freezer RAM not in 8k banks, like the cartridge

emulation, but in 4k banks. The cartridge emulation can access the complete RAM except

for the last 16k, making available a maximum of 496k (banks 0. . . 61).

8.2.2 Flash ROM

The cartridge emulation can access the complete 1MB ﬂash ROM, the freezer can access the

upper 512k thereof. The topmost 64k are reserved for the freezer software, the Oldrunner

OS and the cartridge emulation menu. The topmost 64k (banks 120. . . 127) are currently

used as follows.

1 2 ... 46 47 48 49 50 51 52 53 54 55 56 ... 63 64

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