
June 1997
53
ADVANCE DATA BOOK v2.0
REGISTER DESCRIPTIONS
CL-PS7500FE
System-on-a-Chip for Internet Appliance
Big Endian
In the big endian scheme, the most-significant byte of a word is stored at the lowest-numbered byte, and
the least-significant byte is stored at the highest-numbered byte.
Byte 0 of the memory system should therefore be connected to D[31:24] (data lines 31 through 24).
Load and store are the only instructions affected by the endianism.
7.1.2
Configuration Bits for Backward Compatibility
Two register bits,
PROG32
and
DATA32
, select one of three processor configurations:
1)
26-bit program and data
space
PROG32
LOW, DATA32 LOW
This configuration forces the ARM processor to operate like the earlier ARM processors with 26-bit address
space. The programmer’s model for these processors applies, but the new instructions to access the CPSR and
SPSR registers operate as detailed in the CL-PS7500FE Programmer’s Guide In this configuration, it is impos-
sible to select a 32-bit operating mode, and all exceptions (including address exceptions) enter the exception
handler in the appropriate 26-bit mode.
2)
26-bit program space and 32-bit data
space
PROG32 LOW, DATA32 HIGH
This is the same as the 26-bit program and data space configuration, but with address exceptions disabled to
allow data transfer operations to access the full 32-bit address space.
3)
32-bit program and data
space
PROG32 HIGH, DATA32 HIGH
This configuration extends the address space to 32 bits, introduces major changes in the programmer’s model
and provides support for running existing 26-bit programs in the 32-bit environment.
NOTE:
Do not select the fourth processor configuration, 26-bit data space and 32-bit program space.
Big Endian
HIGHER
ADDRESS
31 24
23 16
15 8
7 0
WORD
ADDRESS
8
9
10
11
8
4
5
6
7
4
0
1
2
3
0
LOWER
ADDRESS
G
Most-significant byte is at lowest address
G
Word is addressed by byte address of most-significant byte
Figure 7-2. Big Endian Addresses of Bytes within Words