Endian Effects For Data Transfers - ARM ARM9TDMI Technical Reference Manual

3.6

Endian effects for data transfers

ARM DDI0145B
The ARM9TDMI supports 32-bit, 16-bit and 8-bit data memory access sizes. The
endian configuration of the processor, set by BIGEND, affects only non-word transfers
(16-bit and 8-bit transfers).
For data writes by the processor, the write data is duplicated on the data bus. So for a
16-bit data store, one copy of the data appears on the upper half of the data bus,
DD[31:16], and the same data appears on the lower half, DD[15:0]. For 8-bit writes
four copies are output, one on each byte lane, DD[31:24], DD[23:16], DD[15:8] and
DD[7:0]. This considerably eases the memory control logic design and helps overcome
any endian effects.
For data reads, the processor will read a specific part of the data bus. This is determined
by the endian configuration, the size of the transfer, and bits 1 and 0 of the data address
bus. Table 3-5 shows which bits of the data bus are read for 16-bit reads, and Table 3-6
shows which bits are read for 8-bit reads.
For simplicity of design, 32 bits of data can be read from memory and the processor will
ignore any unwanted bits.
DA[1:0]
00
10
DA[1:0]
00
01
10
11
Copyright © 1998, 1999 ARM Limited. All rights reserved.
ARM9TDMI Processor Core Memory Interface
Table 3-5 Endian effects for 16-bit data fetches
Little (BIGEND = 0)
DDIN[15:0]
DDIN[31:16]
Table 3-6 Endian effects for 8-bit data fetches
Little (BIGEND = 0)
DDIN[7:0]
DDIN[15:8]
DDIN[23:16]
DDIN[31:24]
Big (BIGEND = 1)
DDIN[31:16]
DDIN[15:0]
Big (BIGEND = 1)
DDIN[31:24]
DDIN[23:16]
DDIN[15:8]
DDIN[7:0]
3-11