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Memory Banks
In this section I
will describe the SVI-328 memory banks, but this also goes for an
expanded SVI-318.
The microprocessor that's in the SVI-328 is the Zilog Z80, that
microprocessor can only address 64KB. Sometimes 64KB isn't enough,
that's why you can switch memory banks in and out. There are 8 banks
devided into 32KB each in the SV-328. Here is a description of the
banks:
FFFFH
8000H |
BANK
02
RAM |
BANK
12
CARTRIDGE ROM |
BANK
22
RAM |
BANK
32
RAM |
7FFFH
0000H |
BANK
01
BASIC ROM |
BANK
11
CARTRIDGE ROM |
BANK
21
RAM |
BANK
31
RAM |
|
Bank No |
Description |
|
01 |
ROM with Microsoft
Extended Basic |
|
02 |
RAM wich is
standard in a SV-328. This bank is used for your Basic programs
and System variabels, approx. 29KB is free when you have started
your computer. |
|
11 |
ROM space for
cartridges. There's nothing here until you insert an cartridge
wich uses this space. |
|
12 |
ROM same as bank 11 |
|
21 |
RAM wich is
standard in SV-328. |
|
22 |
RAM space for
expansion with a SV-807 |
|
31 |
RAM space for
expansion with a SV-807 |
|
32 |
RAM space for
expansion with a SV-807 (intended for use with CP/M v3.0) |
The switching of memory banks is
done with the sound chip (AY-3-8910). There are two 8 bit I/O ports in
the sound chip (port A and B). This has nothing to do with the
production of sound, it's just an extra feature.
Register R7 is a multifunction /ENABLE register wich controls three
Noise/Tone mixers and the two general purpose I/O ports directions. The
direction (input or output) of the two general purpose I/O ports is
determined by the state of bits B7 (port B) and B6 (port A) of R7.
Port A is used for input, so bit 6 should be set to 0.
Port B is used for output, so bit 7 should be set to 1.
Read or writing data to port A or B is done the same way a register is
accessed in the PSG. The register values for the ports are:
Port A = register 14
Port B = register 15
The memory bank switching is connected to port B, as below:
|
Bit |
Description |
|
0 |
ROM Cartridge |
|
1 |
RAM Bank 21 |
|
2 |
RAM Bank 22 |
|
3 |
RAM Bank 31 |
|
4 |
RAM Bank 32 |
|
5 |
Caps Lock LED
on/off? |
|
6 |
ROMEN0? |
|
7 |
ROMEN1? |
Sample source
Setting the correct Input/Output
of Port A and B
; Sample - Set Port A and B I/O
latch EQU 88H
write EQU 8CH
psgreg EQU 07H
LD A,psgreg
OUT (latch) ;select PSG register 7
LD A,$BF ;Port A (in), Port B(out)
OUT (write),A
Switching Bank 31 in/out:
; Sample - bank 31 switch program
latch EQU 88H
write EQU 8CH
read EQU 90H
portb EQU 0FH
LD A,portb
OUT (latch), A ; select PSG register 15 (Port B)
IN A,(read) ; read current value
XOR 00000010B ; xor bit for bank 31
OUT (write),A ; switch bank
Detecting a cartridge in Bank
11:
; Sample - cartridge (bank 11) test program
latch EQU 88H
write EQU 8CH
read EQU 90H
portb EQU 15
LD A,portb
OUT (latch), A ; select PSG register 15 (Port B)
IN A,(read) ; read current value
LD C,A ; store value
AND 11111110B ; bank 11
OUT (write),A ; switch
LD HL,(0) ; address 0 into HL
LD A,L
CP F3 ; is the first byte F3H?
JR NZ,exit
LD A,H
CP 31 ; is the second byte 31H?
JP Z,0 ; yes, jump to the start address
exit LD A,C ; restore value
OUT (write),A ; switch back
RET
Source: Tomas Karlsson
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