Lexicon MPX-1 - Local Address Map; Accessing Memory and I;O Ports

LOCAL ADDRESS MAP
FROM
TO
SIZE
FUNCTION
NOT~S
------------------------------------------------------------------
0000
3FFF
4-16K
RAM
1,4
4000
SFFF 2-8K
EPROM
1,2,5
8000
8001 2
8259A
Registers
8002
1
Set
Interrupts Latch
2
8004 1
DMA
Address Bits 8-15
3
8005 1
DMA Address Bits
16-23 3
8007 1
Interrupt Response Byte
3
COOO
FFFF
16K
External
Window 6
All I/O Ports 256 External
Window 6
Notes: 1. RAM and EPROM exchange locations for power-on-jump.
2. Read Only.
3. Write Only.
4. 4K or 16K depending on chips used.
5. 2K fo~ 2716, 4K for 2732, 8K for 2764.
6. Lower 8 address bits (AO-7) pass through.
ACCESSING MEMORY ON THE BUS
An access to memory that resides on the 5-100 Bus will be performed if the
CPU makes an access to th~ range noted as the external window in the address map
above (addresses COOO to FFFF). The low order 8 bits of the address (AO-7) will
be taken from the CPU's address lines directly, while the high order 16 bits
(A8-23) will come from the address that has been previously written to the DMA
address registers. So the procedure for accessing memory on the bus would be:
1. Write desired AB-15 bits to memory location B004.
2.
Write desired A16-23 bits to memory location BOOS.
3. Perform a memory reference to COXX to FOXX, where XX
represents the low order B bits (AO-7) of the desired address.
If the memory reference to the external window is a read operation, then a
memory
read will occur on the bus. If the memory reference is write operation,
data will be written to the memory on the bus. Note that Ml cycles may also be
executed on the bus meaning that the MPX-1 may actually execute a small amount
of code (less than 256 bytes) directly from the bus.
Note that the high order byte of the external window is insignificant. Any
address in the range COOO to FFFF will be treated identically. For example,
C083, BD83, A9B3, and FFB3 would all access the same external memory location -
XXXX83 , where XXXX are the two bytes from the DMA address registers.
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