Atari 130XE Uživatelský manuál stáhnout pdf (Strana 53)

8.3 Freezer

8.3.2 Mirroring the freezer memory in

When the freezer is activated, the freezer RAM resp. the ﬂash ROM is mirrored to the

area $0000. . . $3FFF. Bank switching is used to choose between 32 RAM banks of 4k each

(a total of 128k) and 64 ROM banks of 8k each (a total of 512k). The memory map in the

Atari looks as follows.

• $0000. . . $0FFF: Fixed 4k RAM bank (bank 31)

• $1000. . . $1FFF: Switchable 4k RAM bank (bank 0. . . 31)

• $2000. . . $3FFF: Switchable 8k ROM bank (bank 64. . . 127)

The 64 ROM banks of 8k each are located at the end of the ﬂash ROM and correspond to

the banks 64. . . 127 of the cartridge emulation. The 32 RAM banks of 4k each are located

at the end of the freezer RAMs and correspond to the banks 48. . . 63 of the cartridge

emulation. The last 4k bank (bank 31) is used for the shadowing of the hardware registers

and is mirrored to the address $0000 in the Atari if the freezer is activated.

8.3.3 Activating the freezer

The freezer logic has 5 internal states.

• Freezer deactivated

• Freezer half activated

• Freezer activation triggered

• Freezer activated

• Freezer temporarily deactivated

After switching the Atari on, the freezer is in the state “deactivated”. In this state and

in the state “half activated” all write access to the custom chip area is stored in the shadow

RAM.

When the “

Freeze

” button is pressed, nothing happens at ﬁrst. Only when the ﬁrst

access to the area $FFF8. . . $FFFF (which contains also the IRQ and NMI vectors) is

performed and the “

Freeze

” button is still pressed, the freezer reacts. It saves the state of

the A2 line (which is required later on to distinguish between IRQ and NMI) and enters

the state “half activated”. In the next clock cycle there are now these two possibilities:

•

If the “

Freeze

” button is no longer pressed or an access to another memory area

than $FFF8. . . $FFFF is performed, the freezer enters the “deactivated” state again.

•

If the “

Freeze

” button is still pressed and there is an access to the memory area

$FFF8. . . $FFFF again, the freezer performs the transition to the state “activation

triggered” as described below.

The freezer deactivates the memory in the Atari and puts the value $21 on the data bus.

This changes the interrupt vectors to the address $21xx, as the CPU is currently trying to

load the high byte of the interrupt vector. The low byte stems from the previous access to

the $FFF8. . . $FFFF area and therefore has the original value.

1 2 ... 48 49 50 51 52 53 54 55 56 57 58 ... 63 64

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