Skip to question
asic.fyi

Hardware interview practice

Verify an in-place DMA merge transaction

EasyHardware-Software IntegrationSystemVerilog

Write the merge oracle and transaction checks for firmware driving a direct memory access (DMA) engine that merges two sorted 16-bit arrays into reserved buffer A.

Starting point

Question code

typedef enum { DMA_OK, DMA_BAD_ARG, DMA_HW_ERROR, DMA_TIMEOUT } dma_status_t;
dma_status_t dma_merge_u16(uint32_t a, uint8_t m, uint8_t a_cap,
                           uint32_t b, uint8_t n, uint32_t timeout);
// A_ADDR, B_ADDR, M, N, A_CAP, CMD{START,ABORT}
// STATUS{BUSY,DONE,ERR,ERR_CODE}; ERR_CODE{BAD_DESC,READ_FAULT,WRITE_FAULT}
Reviewed example

Work through one case

Input
A storage=[1,4,7,_,_,_], m=3, capacity=6; B=[2,4,9], n=3
Expected output
A after DMA=[1,2,4,4,7,9]

The merge is sorted, preserves both copies of 4, and writes only inside the reserved six-element A destination.

What to cover

Requirements

  1. Validate lengths, destination capacity, alignment, address wrap, and nonoverlap before touching registers.
  2. Require A to be a valid destination whenever m+n is nonzero and B to be valid whenever n is nonzero.
  3. Snapshot both active inputs at accepted START and keep every write inside A[0:m+n).
  4. Stop after the first memory fault; map faults and timeout distinctly and suppress late completion after abort or reset.
Continue practicing
Hardware-Software IntegrationVerify a DMA flood-fill transactionFirmware AlgorithmsIntersect redundant-core fault listsConstraintsConstrain a DMA transfer to one 4KB page