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Significance of [MS] and [LS] in big-endian data bus in AHB5 Specification

Pandey

Hi,

As indicated in few previous answers (Link Given below) on the big-endian, I believe that the big-endian type in AHB-Lite Spec is BE32. And further the AHB5 Spec added another type of big-endian as BE8. Hence, the active byte lanes for a 32-bit big-endian data bus should be the same which are described in AHB-Lite (AMBA 3 Spec, Table 6-2) and AHB5 (AMBA 5 Spec, Table 6-3).

However, in the AMBA 5 Spec the table has byte lanes as "Active[MS]" and "Active[LS]". It was not like that in AMBA 3 Spec. What is the significance of MS and LS here? Are they MSB Bytes and LSB Byte?

And just for a quick confirmation, Since, the AHB5 only adds BE8, are the tables "AMBA 3 Spec, Table 6-2" and "AMBA 5 Spec, Table 6-3" same and they are for BE32 only?

Endian about AHB-Lite and AHB5 Specification

How to select endianess in AHB?

For your reference I have attached the table here.

AMBA 3 Spec, Table 6-2: Active byte lanes for a 32-bit big-endian data bus

Transfer Size Address Offset DATA[31:24] DATA[23:16] DATA[15:8] DATA[7:0]
Word 0 Active Active Active Active
Halfword 0 Active Active
Halfword 2 Active Active
Byte 0 Active
Byte 1 Active
Byte 2 Active
Byte 3 Active

AMBA 5 Spec, Table 6-3: Active byte lanes for a 32-bit word-invariant big-endian data bus

Transfer Size Address Offset DATA[31:24] DATA[23:16] DATA[15:8] DATA[7:0]
Word 0 Active[MS] Active Active Active[LS]
Halfword 0 Active[MS] Active[LS]
Halfword 2 Active[MS] Active[LS]
Byte 0 Active
Byte 1 Active
Byte 2 Active
Byte 3 Active

I have one more question - Does AHB system has BE-A (Address invariant) format also? But I believe the AHB Specs do not discuss about them.

Many Thanks,

Manish

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