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Chapter 1 Architectural Overview
when required. Refresh is overlapped in the background with non–DRAM access
when possible.
If a DRAM bank contains video–DRAM rather than conventional DRAM, then
it is possible to perform data transfer to the VDRAM shift register via accesses in the
VDRAM address range. The VDRAM is aliased over the DRAM region. Accessing
the memory as VDRAM only changes the timing of memory control signals such as
to indicate a video shift register transfer is to take place rather than a CPU memory
access.
1.8.3 Virtual DRAM Region
A 16–Mbyte (24 address bit) virtual address space is supported via four map-
ping registers. The virtually addressed memory is divided into 64K byte (16 address
bits) memory pages which are mapped into physical DRAM. Each mapping register
has two 8–bit fields specifying the upper address bits of the mapped memory pages.
When memory is accessed in the virtual address space range, and one of the four
mapping registers contains a match for the virtually addressed page being accessed,
then the access is redirected to the physical DRAM page indicated by the mapping
register.
When no mapping register contains a currently valid address translation for the
required virtual address, a processor trap occurs. In this case memory management
support software normally updates one of the mapping registers with a valid mapping
and normal program execution is restarted.
Only DRAM can be mapped into the virtual address space. The address region
supports functions such as image compression and decompression that yield lower
overall memory requirements and, thus, lower system costs. Images can be stored in
virtually addressed space in a compressed form, and only uncompressed into physi-
cally accessed memory when required for image manipulation or output video imag-
ing.
1.8.4 PIA Region
The Peripheral Interface Adapter (PIA) region is divided into six banks, each of
24–bit address space. Each bank can be directly attached to a peripheral device. The
control registers associated with the region give extra flexibility in specifying the
timing for signal pins connecting the microcontroller and PIA peripherals. The PIA
device–enable and control signals are again provided on–chip rather than in external
support circuitry.
When
externa
l DMA is utilized, transfer of data is always between DRAM or
ROM space and PIA space. More on DMA follows.
1.8.5 DMA Controller
When an off–chip device wishes to gain access to the microcontroller DRAM, it
makes use of the Direct Memory Access (DMA) Controller. On–chip peripherals can