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AXI Wrap Bursts

In case of wrapped bursts, we need to calculate first the Aligned_Address, using:

Suppose start address is 55, assuming 32 - bit bus, burst length of 4

Aligned_Address = (INT(Start_Address / Number_Bytes) ) * Number_Bytes;

The value is :: 52 or 56 i.e. do we have round to lower or upper value.

Then we calculate the wrap boundary, using
Wrap_Boundary = ((INT(Start_Address / (Number_Bytes * Burst_Length))) * (Number_Bytes * Burst_Length);

What does this wrap boundary actually indicate,
1. The address from where wrapping will take place.
2. The address value after wrap.

Also, if anyone can let me know in a step-wise manner how the address are calculated using the same scenario above, would be great.

Hope to see the replies soon.

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0 Peter Harris over 12 years ago

Note: This was originally posted on 1st July 2009 at http://forums.arm.com

As I mentioned before, your example is not a valid AXI transfer - it can never happen.

The formula in the spec if for calculating the Aligned address in the wrap, not for aligning the start address - that must be aligned by the master making the access.

For a 4-byte beat your start address must be 4-byte aligned. 55 is not. To repeat what I said before:

[snip]
Start_Address needs to be aligned on the beat-size, so for a 32-bit beat it needs 4-byte aligned, 16-bit beat needs to be 2-byte aligned, etc. In this case your start address (55) is not a valid Start_Address for a 32-bit beat - ..., 52, 56, 60, 64, ... would be OK for example.
[/snip]

However, assuming a start address of 56 the following calculation would apply:

Burst Len = 32-bit beat * 4 beats = 16 bytes

Aligned address:
= INT(56 / 16) * 16
= INT(3.5) * 16
= 3 * 16
= 48

Wrap boundary:
= highest byte accessed in burst
= 48 + 16 -1
= 63

Access Pattern:
= 56, 60, 48, 52
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0 Peter Harris over 12 years ago

Note: This was originally posted on 1st July 2009 at http://forums.arm.com

As I mentioned before, your example is not a valid AXI transfer - it can never happen.

The formula in the spec if for calculating the Aligned address in the wrap, not for aligning the start address - that must be aligned by the master making the access.

For a 4-byte beat your start address must be 4-byte aligned. 55 is not. To repeat what I said before:

[snip]
Start_Address needs to be aligned on the beat-size, so for a 32-bit beat it needs 4-byte aligned, 16-bit beat needs to be 2-byte aligned, etc. In this case your start address (55) is not a valid Start_Address for a 32-bit beat - ..., 52, 56, 60, 64, ... would be OK for example.
[/snip]

However, assuming a start address of 56 the following calculation would apply:

Burst Len = 32-bit beat * 4 beats = 16 bytes

Aligned address:
= INT(56 / 16) * 16
= INT(3.5) * 16
= 3 * 16
= 48

Wrap boundary:
= highest byte accessed in burst
= 48 + 16 -1
= 63

Access Pattern:
= 56, 60, 48, 52
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