07Verification

Match a status word format

Problem

Intended behavior. A peripheral publishes a 16-bit status word with this documented layout:

Bits	Field	Width
15	`valid`	1
14–12	`channel`	3
11–0	`count`	12

StatusWord packs the Status input in into word and unpacks the flat input raw into the Status output out. With valid = 1, channel = 5, and count = 0x123, word must be d123; raw = a00f must read back as valid = 1, channel = 2, count = 00f.

Current behavior. The starter compiles and runs, but both printed rows disagree with the table.

Task. Find the cause and fix it so that packing and unpacking both follow the table. Keep the port names, the field names, and the field widths.

Read the starter

Both conversions are one-liners, asUInt and asTypeOf, and neither mentions a bit position. Ask where they get the layout from, then compare that source with the table, field by field, from the top bit down.

Predict before running

Write the 16 bits of the starter's packed word for the sample input by laying the fields out in the order the code implies. Do the same for a00f in the unpack direction. Your predictions should match the starter's 891d and its channel=7 count=401.

Run and explain

891d is 1 followed by 0001_0010_0011 and then 101: the count sits directly under valid, and the channel occupies the bottom three bits. The unpack row is the same layout read backwards. One declaration feeds both conversions, so one mistake breaks both. After the fix, the rows read d123 and valid=1 channel=2 count=00f.

Completion check

Every combination of field values packs to the documented bit positions.
Every 16-bit raw value unpacks into the documented fields.
Packing and unpacking agree with each other; fixing only one direction is incomplete.

Progressive hints

Hint 1

asUInt and asTypeOf do not know the peripheral's format. They read the bit layout from the Bundle declaration and nowhere else.

Hint 2

The first declared field takes the most significant bits. Compare the order of the second and third fields in Status with the table. A patched Cat in the pack direction leaves the unpack direction wrong.

Concept

A Bundle that describes an external format is a specification: its field order is the bit layout. Correcting the declaration fixes every conversion that uses it, while manual slicing at individual use sites lets the two directions drift apart.

Transfer experiment

Suppose the format moved valid to bit 0 and shifted the other fields up by one. Predict the packed word for the sample input, and state which declaration change produces it.