ARM R4F Restrictions on AXI transfers, Strongly Ordered and Device transactions, Address20

Level Two Interface

9.3.1Restrictions on AXI transfers

The Cortex-R4 AXI master interface applies the following restrictions to the AXI transactions it generates:

•A burst never transfers more than 32 bytes.

•The burst length is never more than 8 transfers.

•No transaction ever crosses a 32-byte boundary in memory. See AXI transaction splitting on page 9-16.

•FIXED bursts are never used.

•The write address channel always issues INCR type bursts, and never WRAP or FIXED.

•WRAP type read bursts, see Linefills on page 9-13:

—are used only for linefills (reads) of Cacheable Normal non-shared memory

—always have a size of 64 bits, and a length of 4 transfers

—always have a start address that is 64-bit aligned.

•If the transfer size is 8 bits or 16 bits then the burst length is always 1 transfer.

•The transfer size is never greater than 64 bits, because it is a 64-bit AXI bus.

•Instruction fetches, identified by ARPROT[2], are always a 64 bit transfer size, and never locked or exclusive.

•Transactions to Device and Strongly Ordered memory are always to addresses that are aligned for the transfer size. See Strongly Ordered and Device transactions.

•Exclusive and Locked accesses are always to addresses that are aligned for the transfer size.

•Write data is never interleaved.

•In addition to the above, there are various limitations to the ID values that the AXI master interface uses. See Identifiers for AXI bus accesses on page 9-4.

9.3.2Strongly Ordered and Device transactions

A load or store instruction to or from Strongly Ordered or Device memory always generates AXI transactions of the same size as implied by the instruction. All accesses using LDM, STM, LDRD, or STRD instructions to Strongly Ordered or Device memory occur as 32-bit transfers.

LDRB

Table 9-4shows the values of ARADDRM, ARBURSTM, ARSIZEM, and ARLENM for a Non-cacheable LDRB from bytes 0-7 in Strongly Ordered or Device memory.

				Table 9-4 Non-cacheable LDRB
		Address[2:0]	ARADDRM	ARBURSTM	ARSIZEM	ARLENM
		0x0 (byte 0)	0x00	Incr	8-bit	1 data transfer
		0x1 (byte 1)	0x01	Incr	8-bit	1 data transfer
		0x2 (byte 2)	0x02	Incr	8-bit	1 data transfer
		0x3 (byte 3)	0x03	Incr	8-bit	1 data transfer
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