HP 330 Service Information Manual page 95

Model 330 Processor Board
Introduction
The Model 330 processor board has the following major parts:
• 16.17 MHz MC68020CPU.
• 16.67 MHZ MC68881 Floating-Point Coprocessor.
• 16.67 MHz MC68851 Memory Management Unit
• 6840 Timer.
• Boot ROMs.
• Self-Test LEDs.
• 4 Mbytes of RAM.
A comparison between both processor block diagrams shows other differences which are the
Paged Mernory Management Unit and 4 Mbytes of RAM. Refer to Figure 3-16 for the following
descri ptions.
Central Processing Unit
An MC68020 CPU is also used on the Model 330 processor board. It is clocked at 16 MHz.
Memory Management Unit
The Model 330 processor board uses the MC68851 MMU for its memory management. No cache
is used on the Model 330 processor board.
l\1MU connections to the CPU are through the coprocessor interface. All accesses to registers
within the PMMU are done by use of coprocessor instructions rather than memory
mapp~
registers. Page size is 4 Kbytes or larger. The TLB is a 64-entry CAM containing the most
recently used translations. When a miss in the TLB occurs the MMU requests the bus from
the MC68020 and conducts its own table walks. When the translation is retrieved the bus is
returned to the MC68020 which re-runs the cycle with the correct translation contained within
the PMMU. The PMMU can address the 8 Mbytes of physical address space supported on
l\1odel 330 computers.
For more information on tables within the PMMU consult the MC68851 Users Guide.
Functional Description
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