Epson ARM.POWERED ARM720T - Manual (Page 73)

4: Instruction and Data Cache
ARM720T CORE CPU MANUAL EPSON 4-1
4 Instruction and Data Cache
This chapter describes the instruction and data cache. It contains the following sections:
4.1 About the instruction and data cache ....................................................... 4-1
4.2 IDC validity ................................................................................................ 4-2
4.3 IDC enable, disable, and reset................................................................... 4-2
4.1 About the instruction and data cache
The cache only operates on a write-through basis with a read-miss allocation policy and a
random replacement algorithm.
4.1.1 IDC operation
The ARM720T contains an 8KB mixed Instruction and Data Cache (IDC).
The cache comprises four segments of 64 lines each, each line containing eight words. The IDC
is always reloaded a line at a time. The IDC is enabled or disabled using the ARM720T Control
Register and is disabled on HRESETn.
Note:
The MMU must never be disabled when the cache is on. However, you can enable
the two devices simultaneously with a single write to the Control Register (see
Control Register on page 3-4).
4.1.2 Cachable bit
The C bit determines if data being read can be placed in the IDC and used for subsequent read
operations. Typically, main memory is marked as cachable to improve system performance,
and I/O space is marked as noncachable to stop the data being stored in the ARM720T cache.
For example, if the processor is polling a hardware flag in I/O space, it is important that the
processor is forced to read data from the external peripheral, and not a copy of the initial data
held in the cache. The cachable bit can be configured for both pages and sections.
Cachable reads (C=1)
A line fetch of eight words is performed when a cache miss occurs in a cachable area of memory,
and it is randomly placed in a cache bank.
Note:
Memory aborts are not supported on cache line fetches and are ignored.
Uncachable reads (C=0)
An external memory access is performed and the cache is not written.