Interview Question · Concept
AXI Response Codes
The Question
The interviewer asks
What responses can an AXI subordinate return? How many responses do a 4-beat write and a 4-beat read get?
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Reveal the Model Answer
OKAY means normal success, and EXOKAY means a successful exclusive access only. SLVERR means the access reached a subordinate that failed it. DECERR means no subordinate was found at the address. A write gets one response on B after its last beat. A read gets one RRESP with every beat, so a 4-beat read has four, and they may differ. An error never shortens a burst.
A pill is one read beat; RRESP 0 is OKAY, 2 is SLVERR.
- Cycle 0
The request is accepted: four beats of four bytes from 0x3000.
- Cycle 3
Beat 0 arrives with RRESP 0, OKAY. The data, a, is valid.
- Cycle 5
Beat 2 is in the dead bank: RRESP 2, SLVERR. RDATA is not valid, but the manager still accepts the beat.
- Cycle 6
Beat 3, also SLVERR, carries RLAST: the burst is finished.
- Cycle 7
Four beats, four responses, one RLAST. An error changes what the data means, never the number of transfers.
Each read transfer carries its own response.
Self-Check
Tick each key point you covered in your answer.
What Interviewers Listen For (Fresher)
A good fresher answer lists the four response codes, with one line on what each means.
What Interviewers Listen For (Junior)
A good junior answer knows a write gets one B response but a read gets one RRESP per beat. It knows every beat still completes after an error.
What Interviewers Listen For (Mid-Level)
A good mid-level answer says who makes SLVERR and who makes DECERR. It knows EXOKAY on a normal access breaks the protocol.
Cross-Questions
Where the interviewer goes next. Open a node to practise that follow-up.
What IfProve ItCompareScale It
- AXI Response Codes
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What IfAn Error in the Middle of a BurstL1 Junior
A 4-beat write gets SLVERR. Which of its bytes were written?
2:30You have two and a half minutes. Speak or type, then press Get Feedback under your answer. No credits needed.
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Reveal the Model Answer
You cannot tell from the response. SLVERR on B covers the whole write. The spec says the location might not be fully updated, so some beats may have landed and some not. Lesson P2.13 showed a write that returned SLVERR with bytes 0a and 0b already in memory. Software treats the whole buffer as suspect.
Self-Check
Tick each key point you covered in your answer.
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What IfAn Error on One Beat of a ReadL1 JuniorL2 Mid-Level
What if beat 2 of a 4-beat read gets SLVERR? Does the burst stop there?
2:30You have two and a half minutes. Speak or type, then press Get Feedback under your answer. No credits needed.
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Reveal the Model Answer
No. The subordinate still sends all four beats, up to RLAST, and the manager takes them. Each read beat carries its own response, so beats 1, 3 and 4 may be OKAY. Only the data of beat 2 is suspect.
Self-Check
Tick each key point you covered in your answer.
Spec Misquote
"After SLVERR the subordinate can stop sending beats."
The request fixes the number of beats. A short burst leaves the manager waiting for RLAST forever.
Source AMBA AXI Protocol Specification IHI 0022L §A3.3.2 Read response
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CompareSLVERR Against DECERRL1 JuniorL2 Mid-Level
A read returns DECERR. Who generated it, and how is that different from SLVERR?
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Reveal the Model Answer
DECERR normally comes from the interconnect. The address decoded to no subordinate, so the access never reached a target. SLVERR comes from a subordinate that received the access and could not complete it. Examples are a write to a read-only register, or an access to a powered-down block. For debug, DECERR points at the address map; SLVERR points at the target.
Self-Check
Tick each key point you covered in your answer.
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Scale ItDesigning the Default SubordinateL2 Mid-LevelL3 Senior
You own the interconnect. What does your default subordinate have to do with a 16-beat read to an unmapped address?
2:30You have two and a half minutes. Speak or type, then press Get Feedback under your answer. No credits needed.
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Reveal the Model Answer
Complete the transaction like any subordinate. Accept the request, and return all 16 beats with DECERR on each and RLAST on the last. Echo the ID, and obey the same handshake and ordering rules. For writes, it must accept every beat up to WLAST before one DECERR on B. A default subordinate that answers with one beat hangs a manager waiting for RLAST.
Self-Check
Tick each key point you covered in your answer.
What Interviewers Listen For (Mid-Level)
A good mid-level answer returns a full-length response with DECERR on every beat.
What Interviewers Listen For (Senior)
A good senior answer adds that the ID is echoed back and same-ID order is kept against real subordinates. It asks for tests that hit holes in the address map.
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Prove ItSolve P2.18.1: Catch Every Broken SubordinateSimulator-graded
Show it, don't just say it: this Silicon Lab problem is graded by the simulator.
What People Get Wrong
Common wrong answers and why they fail.
Misconception
"After SLVERR on beat 2 the subordinate stops; the read ends early."
Every beat of the length must complete, whatever the response. Only the meaning of the data changes. Lesson P2.13 measured OKAY OKAY SLVERR SLVERR across four beats.
Source AMBA AXI Protocol Specification IHI 0022L §A3.3.2 Read response; §A3.1.2 Length attribute
Spec Misquote
"Each write beat gets its own response, like reads."
A write has one completion response on B for the whole transaction.
Source AMBA AXI Protocol Specification IHI 0022L §A3.3.1 Write response