Apple Logic Board Información técnica descargar pdf (Pagina 72)

II. CONTROLLING THE SATURN 64K and 128K RAM BOARD

The Saturn 64K and 128K boards are controlled using 16

locations in the range $CONO — $CONF, where N = 8 + Saturn board

slot #. Thus for slot zero operation, the control addresses are

in the range $C080 to $C08F. These locations are in the Apple’s

peripheral I/O space as described in Apple II reference manual,

p. 80. They are used by the Saturn 64K and 128K boards to control

the following functions:

1. Select the Saturn RAM board or the Apple main board

ROM.

2. Select the desired 16K bank to access.

3. Determine which 4K bank (A or B) within each 16K bank is

to be used.

4. Write enable or write protect RAM.

In order to set the Saturn 64K or 128K board in a given

state, a memory access to the particular location corresponding

to the desired state is all that is required. This can be accom-

plished by either reading or writing to the location. From

BASIC, a POKE or PEEK will accomplish this memory access. A

machine language load or store will also serve the same purpose.

The lower 4 bits of the control address selects the mode of

operation of the Saturn 64K and 128K boards, as well as

determines which 16K bank is to be accessed.

A. When address line 2 is low (A2=0) state selection is enabled.

In this case address lines 0, 1 and 3 determine the state which

is set as follows.

1. Address line 0 determines whether the board will be write

enabled or write protected. (AO = 0 for write protect; AO = 1 for

write enable)

2. Address lines 0 and 1 are used to determine whether the

main board ROMs are accessed, or the RAM on the Saturn board.

3. Address line 3 determines which 4K bank within the

selected 16K bank to use. (A3 = 0 for 4K bank A; A3 = 1 for 4K

bank B)

B. On the other hand when address line 2 is high (A2=l) 16K Bank

selection is enabled. The address lines 0, 1 and 3 determine

which 16K bank is selected.

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Apple Logic Board Información técnica Pagina 72