Apple Logic Board Información técnica descargar pdf (Pagina 74)

Using the appropriate mode combination (previous table), any

given byte of memory on the Saturn RAM board can be accessed. In

fact, in several modes (CON1, CON9) the main board ROM’s are

accessed during a read from $DOOO — $FFFF, whereas the RAM on the

Saturn RAM board is written to during a write operation. This

enables one to write to the RAM while running BASIC in ROM which

occupies the same address space. This is possible since BASIC is

being read from ROM while the program may in fact be writing to

RAM (via POKES).

Note Concerning Write Enabling the RAM:

In order to enable the Saturn 64K or 128K RAM board for

writing to RAM, the corresponding control address (for mode

desired) must be accessed twice. This is required for any of the

write enable modes. If the board is already in a write enabled

state (e.g. $CONl), and another mode is desired which is write

enabled as well (e.g. $CON3), then only one access to the control

address corresponding to this new mode is required. If on the

other hand, the board is in a write protected state (e.g. C0N0)

prior to the change, two access to the control address are

required to write enable the board.

III. SATURN 64K and 128K RAM BOARD MEMORY USE

A. RELOCATED DOS

The relocated DOS occupies 16K Bank 2 and 4K Bank 2A, using

the full 12K of memory space from $DOOO — $FFFF, for DOS, monitor

routines and associated file buffers. This bank is enabled using

control address $CON3 and Bank select address $CON5. The 4K Bank

2B is not used by the relocated DOS.

B. ALTERNATE BASIC

When the alternate BASIC (INTEGER for an Apple II plus;

Applesoft for an Apple II) is present in the Saturn RAM board, it

occupies 16K Bank 1 and 4K Bank lA, and is enabled for reading

using control address $CONO and bank select $CON4. The 4K Bank

lB is not used by this BASIC.

C. LED INDICATORS:

The Saturn 64K and 128K RAM boards contain 4 light emitting

diodes (LEDs) which provide some visual indication of the state

of the card. Three of the LEDs indicate which bank is selected

and the fourth whether the board is read enabled (RAM READ state)

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Apple Logic Board Información técnica Pagina 74