ADVANCE INFORMATION
VCT 38xxA
5.8. Memory Banking
Banking
Register
*D0 ... D7
A15 ... A19
A15 ... A19
Interrupt
Controller,
DMA
Address
Decoder,
Memory,
I/O
65C02
Logic
*A15
A0 ... A14
A0 ... A14
*A0 ... A14
*Processor internal Bus
Fig. 5–5: Block diagram of Memory Banking
The 8-bit processor W65C02 only allows access to
64 kByte of memory space. To allow access to the
expanded memory range above 64 kByte, a specific
banking hardware is implemented. The physical
address range above 32 kBytes (A15 = 1) is separated
into several banks of which only one at a time is
enabled and selected by the Banking register (BR),
which is programmable as any other standard periph-
eral register by writing the desired value into its spe-
cific address. The content of the BR is also readable,
so the software may check the current bank at any
time. The applied software is responsible to program
the BR with the correct bank number at the right time.
Since the upper 32 kBytes range is switched immedi-
ately after programming the BR, correct function is not
guaranteed if it is changed by a program sequence
running in a switched bank. BR settings need to be
done in the lower 32 kBytes (A15 = 0), which is the
non-switchable master bank (bank 0).
5.8.1. Banking Register
37: 1F0F
38: BR
39: Banking Register
bit
7
6
0
5
0
4
0
3
2
BN
0
1
0
1
r/w
reset
0
0
0
BN
r/w:
Bank Number
number of 32 kByte memory bank
Setting BN = 0 should be avoided because it will mirror
the non-switchable master bank (bank 0) into the
upper 32-kByte area (A15 = 1). RAM, I/O pages and
reserved addresses may be manipulated unintention-
ally.
RESET initializes BN = 1 to read control byte and reset
vector from bank 1. Also, interrupt vectors have to
reside in bank 1, because the Interrupt Controller gen-
erates the appropriate address of bank 1, but it does
not change the contents of the BR. Interrupt functions
have to reside in the non-switchable master bank
(bank 0). Otherwise,they need to be in each used
bank, because after getting the vector the unchanged
contents of the BR determine the current bank which is
valid if A15 is “1”.
Micronas
95
MICRONAS [ MICRONAS ]
