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Chapter 9

TECHNICAL INFORMATION

I. ADDRESSING THE SATURN 64K AND 128K RAM BOARDS

The Saturn 64K RAM board contains 64K bytes(and the 128K RAM

board, 128K bytes) of RAM memory, divided into 16K byte banks.

These banks are selected independently of each other, and in many

respects appear as four 16K RAM boards (for the 64K RAM board) or

eight 16K RAM boards (for the Saturn 128K board) occupying a

single slot.

All of the 16K banks occupy the same memory space as the

Apple main board ROMS, from $DOOO to $FFFF. In this respect,

they share the same memory space as the ROMS. At any given time,

either the main board ROMS are accessed, or one of the 16K banks,

but not both. Since the ROM space from $DOOO to $FFFF is only

12K long, and each bank on the Saturn RAM board is 16K bytes in

size, only 12K each of these banks can be accessed at any given

time. This is because the 4K memory space from $COOO to $CFFF is

used by the Apple II for I/O and cannot be shared by the RAM on

the Saturn 64K or 128K boards.

In order to access the full 16K bytes of each bank on the

Saturn board, the lower 4K of the RAM board address space is used

twice. Each 16K bank of the Saturn RAM board is divided into two

4K banks occupying the memory space from $DOOO to $DFFF, and one

8K block from $EOOO to $FFFF. Within a given 16K bank, one may

select either of the two 4K banks to occupy the space from $DOOO

to $DFFF.

One has access to the full 64K on the Saturn 64K board (128K

on the Saturn 128K board) by selecting it in 12K segments. To

define a given segment, two things must be designated.

1. The 16K bank to be accessed.

2. The 4K bank to be used within this 16K bank.

The following convention has been established to describe the

Saturn 64K board. The four 16K banks are denoted 16K Bank 1, 16K

Bank 2, 16K Bank 3 and 16K Bank 4. Within each 16K bank, the

4K banks are described as 4K Bank A and 4K Bank B. In order to

distinguish the 4K banks within 16K Bank 1 from those in 16K Bank

2, 16K Bank 3 or 16K Bank 4, the 4K bank designation is preceded

by 1, 2, 3 or 4, indicating the 16K bank to which it belongs.

Thus, 4K Bank lA describes the first 4K bank within 16K Bank 1.

The Saturn 128K RAM board is described in the same manner as

the 64K board, with the exception that it contains eight 16K

banks, rather than 4. These banks are denoted 16K Bank 1 through

16K Bank 8.

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