Megabyte Memory Mapping Operation - HP 9020 Service Manual

2-26
Theory of Operation
Memory Mapping
The Memory Controller, as on other RAM boards, has the memory mapping capability of translat-
ing logical addresses on the MPB to physical addresses in RAM. The difference is the controller
output (Figure 2-17). There is only one mapper Cam on the 1 Megabyte board. The mapper CAM
uses bit zero for the Map Out bit to indicate whether the memory card is capable of storing data or is
mapped out. Bits 3 through 11 are used for determining whether this 1 Megabyte board is being
addressed by the logical memory on the MPB. If the MPB logical address bits 3 to 11 match the bits
in the mapper CAM, the operational transaction is for this board. As all memory chips are addres-
sed without the requirement for Chip Select signals or block addressing, the nine bits of X, and the
nine bits of Y address are all that are required to locate the desired information in the RAM chips.
The MC controls when the chips are addressed by output of the NRAS and NCAS control signals,
along with the output of the X or Y address on the Memory Address Bus.
MPB
, -____________________________
~A,
______________________________
~
r ,
I~
LOGICAL MEMORY ADDRESS
-------------1
(27 BITS)
I
2 3 11 12 29 30 31
9
9
{
MAPPER
CAM
1 1
\
Y
I
X AND Y PHYSICAL
ADDRESS
BOARD ADDRESS AND
{
...
__
~
___
2~3
___________________
1_1~1_2
_______________________________
3~1
X INTERLEAVE CONFIGURATION X - - - - - - - - - - - - - - - X
X-DON'T CARE MAPPER CAM DATA FORMAT
Figure 2-17. 1 Megabyte Memory Mapping Operation