Commodore Amiga A500 Technical Reference Manual page 42

Further analysis of the
BD1 5-BD1 2
equations will show that almost
all addresses put out ones; however, remember that most of the nib-
bles are inverted because the spec says they have to be. The inversion
makes it possible to implement the codes in active low PALS; it isjust
a cost reduction.
Analysis of the equations shows that the only nibbles (we don't care
about above
HEX 80)
outputting any zeros are:
To interpret this code, we need to remember that the spec says that
all nibbles get inverted except
00,02,40,
and
42.
So our new table
looks like this:
And all the other nibbles that were ones are now inverted to zeros.
To illustrate, let's look at what these codes mean:
Nibble
Data
00 l 02 1100 0001
I 0 0 1
=
64
kilobytes, the smallest size that
can be requested.
0
=
There are no more PlCs on this physical
board. It is possible to put more than
one PIC on a physical board, but in
most cases (including this one), we don't.
0
=
This board does not have any lnit or
diagnostic code.
0
=
Don't link into memory free list, since
the processor might try to use it
and it is only
16
kilobytes masquerading to
the system as 64 kilobytes.
I
1 1
=
Required by the spec.
04 106 0000 0110
=
Product number
=
6
10112 0000 0001
=
High byte of manufacturer's number