Commodore VIC-20 Programmer's Reference Manual page 260

NMJ—When this line is grounded, it causes a VIC WARM START
(just like RUN STOP-RESTORE).
Address Bus—The address bus controls what memory loation or
I/O device the VIC will read from or write to. Only 14 bits appear on
the connector, even though the address bus size is 16 bits,
because two bits are decoded into the BLOCK and I/O signals.
Data Bus—The data bus is used by the VIC to move data to or from
memory or I'O devices.
I/O Signals—These two signals can be used to add additional I/O
devices to the VIC. The IEEE adapterf rom Commodore uses these
signals.
Read/Write Signals—These signals inform the memory or the
device being addressed whether the VIC wants to write data or read
data. If the signal is high, a read is expected. If the signal is low, a
write es desired.
There are two RA/V signals available on the expansion port. One
(CR/W) is connected to the 6502. The other (VR/W) is connected to
the VICchip. Memory expansion will normally use the VR/Wsignal.
Other devices may need the CRW signal.
WHAT HAPPENS WHEN MEMORY IS
EXPANDED
The VIC comes with 5K of random access memory (RAM)
located from 0 to 1023 (SOOOO to S03FF) (or operating system use,
4096-8192 (S1000 to S1FFF) which is BASIC program area, and
from 38400 to 38911 (S9600 to S97FF) which is COLOR memory
area.
When additional memory is added the VIC screen location, color
memory location, or the start of BASIC might change.
Start of Start of Start of Start of
Added MemoryScreen Color Memory BASIC
1024-4095 7680 ($1 E00) 38400 (S9600) 1024 (S0400)
8192 on up 4096 ($1000) 37888 (S9400) 4608 (S1200)
(S2000-3FFF)
(S4000-5FFF)
(36000-7FFF)
The VIC has 2 areas to add additional memory—a 3K space from
1024 to 4095 (S0400 to $0FFF) and a 24K section from 8192 to
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