23Verification

Read a request field

Diagnose the starter

Intended behavior. A Request keeps an eight-bit starting address, a four-bit transfer length, and a read/write flag in named fields. lastAddr must add the zero-extended length to req.addr; arithmetic remains eight bits, so a sum past 8'hff wraps around.

Current behavior. The starter compiles, but lastAddr packs the whole request and assumes its low eight bits are the address. Those bits actually mix parts of several fields, so changing length or write can corrupt the address used by the calculation.

Fix. Read the starting address through the struct field intended to hold it. Keep the Request declaration, constructor, and function signatures unchanged.

Read and predict

Read the struct declaration and constructor before lastAddr. For a fixed address, predict whether toggling write should affect the result, then explain why slicing pack(req) violates that expectation.

mkTop builds a request with address 8'h40, length 3, and write set, then prints addr and lastAddr. The first declared field occupies the high end of the packed value. Work out which field bits land in the low eight positions and predict the starter's lastAddr before running it once.

Repair path

Change only the source of the starting address. Keep the existing explicit extension for length so the addition occurs at eight bits. After the repair, the public run should print hexadecimal 43 for lastAddr. Explain why that value no longer depends on write, then try cases that change length and write independently, including one sum that wraps.

Completion contract

Compute the result from the named addr and length fields.
Make the result independent of the write field.
Preserve eight-bit wraparound.
Keep Request, makeRequest, and both function signatures unchanged.
Work across field combinations and the 8-bit boundary; matching the packed layout of one request is insufficient.

Progressive hints

Hint 1

The dot operator selects a struct field by name, without exposing where its bits happen to sit in the packed representation.

Hint 2

The length is already read as req.length. The starting address has a field that can be read in exactly the same way.

Concept

A struct gives each part of a value a stable name. deriving (Bits) also gives the complete struct an encoding, but bit positions are a storage detail rather than the interface between fields and logic. Named selection keeps the source correct if field widths or declaration order later change.

Transfer experiment

Imagine the fields are reordered or a new flag is inserted. Explain why named selection survives both changes while a fixed packed slice must be audited.