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Late clarifications, from public review #34

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Aug 23, 2024
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Late clarifications, from public review #34

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6ad1528
remove reference to buffer head, which is ambiguous, per jf feedback
bcstrongx Aug 19, 2024
8986f32
clarify WRPTR treatment on returns in RASEMU
bcstrongx Aug 19, 2024
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2 changes: 1 addition & 1 deletion body.adoc
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Expand Up @@ -688,7 +688,7 @@ The CC value is only valid when the Cycle Count Valid (CCV) bit is set. If CCV=
When the optional `mctrctl`.RASEMU bit is implemented and set to 1, transfer recording behavior is altered to emulate the behavior of a return-address stack (RAS).

* Indirect and direct calls are recorded as normal
* Function returns pop the most recent call, by invalidating logical entry 0 (by setting `ctrsource`.V=0) and decrementing the WRPTR, such that (invalidated) logical entry 0 moves to logical entry depth-1, and logical entries 1..depth-1 move to 0..depth-2.
* Function returns pop the most recent call, by decrementing the WRPTR then invalidating the WRPTR entry (by setting ctrsource.V=0). As a result, logical entry 0 is invalidated and moves to logical entry depth-1, while logical entries 1..depth-1 move to 0..depth-2.
* Co-routine swaps affect both a return and a call. Logical entry 0 is
overwritten, and WRPTR is not modified.
* Other transfer types are inhibited
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2 changes: 1 addition & 1 deletion intro.adoc
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Expand Up @@ -15,7 +15,7 @@ _Software may opt to use a depth less than the maximum supported in order to red

Only qualified transfers are recorded. Qualified transfers are those that meet the filtering criteria, which include the privilege mode and the transfer type.

Recorded transfers are inserted to the head of the buffer, while older recorded transfers may be overwritten once the buffer is full. Or the user can enable RAS (Return Address Stack) emulation mode, where only function calls are recorded, and function returns pop the last call record. The source PC, target PC, and some optional metadata (transfer type, elapsed cycles) are stored for each recorded transfer.
Recorded transfers are inserted at the write pointer, which is then incremented, while older recorded transfers may be overwritten once the buffer is full. Or the user can enable RAS (Return Address Stack) emulation mode, where only function calls are recorded, and function returns pop the last call record. The source PC, target PC, and some optional metadata (transfer type, elapsed cycles) are stored for each recorded transfer.

The CTR buffer is accessible through an indirect CSR interface, such that software can specify which logical entry in the buffer it wishes to read or write. Logical entry 0 always corresponds to the youngest recorded transfer, followed by entry 1 as the next youngest, and so on.

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