IoT Fundamentals · Study deck

Bitwise Operations and Endianness

Picture a configuration write that enables an alarm but also changes the sensor range.

Physics Phoebe is your guide for this deck.

databitwiseoperations

Physics Phoebe, the module guide, in a scene from this chapter.

After studying this chapter

Learning objectives

You will be able to:

Explain: This supports the bitwise chapter's contract argument: multi-byte fields need an explicit byte order beside width, signedness, scale, and version before shifts or masks can produce trustworthy values.
Explain: A row of light switches is the other half of the picture: a mask is the list of switches you are allowed to touch, leaving the rest alone.
Explain: It should apply the documented masks, test each flag, and leave reserved bits untouched so future firmware can add meanings without breaking old readers.
Explain: Firmware means the program stored on a device to control its hardware.

Real IoT code uses AND to read, OR to set, XOR to toggle, and shifts to pack or extract bit fields.

Major section

In 60 Seconds

The compact byte hides several settings that must be kept separate.

Firmware means the program stored on a device to control its hardware.
A payload means the part of a message that carries the application data.
The safe habit is to treat byte order, bit positions, width, scaling, units, and reserved bits as one documented contract.

Major section

Start With the Story

You will decode multi-byte fields, extract packed flags, and update selected register bits without disturbing their neighbours.

Several flags first share one byte, so an order or mask error can reach a valid but unintended bit.
The unchanged neighbours in the last panel are the decisive test: the operation modified its target without corrupting unrelated state.

Major section

Decode Bytes by Contract

A payload may carry a two-byte temperature, a four-byte timestamp, and a one-byte status register.

To decode it safely, both sides must agree on byte order, bit positions, scaling, and field width.
Two ideas do most of the work.
AND checks or extracts, OR sets, XOR toggles, and shifts move fields into place.

The value 0x12345678 in big-endian (12 34 56 78) and little-endian (78 56 34 12) byte order.

Major section

Decode Bytes by Contract (continued)

The core rule is precise: bits are numbered positions inside a byte, and a mask selects the positions you care about.

Any multi-byte field needs an agreed byte order before it crosses a device, gateway, file, or network boundary.
The same ink can read as two different numbers.
A row of light switches is the other half of the picture: a mask is the list of switches you are allowed to touch, leaving the rest alone.

Major section

Decode Bytes by Contract (continued)

Big-endian places the most significant byte first, so hexadecimal 12 34 becomes $18\times256+52=4660$ .

For example, a battery-powered sensor might send one status byte where bit 0 means the temperature sample is valid, bit 2 means low battery, and bit 4 means the gateway link is ready.
The decoder should not judge the decimal value of the whole byte.
The network-byte-order note shows how canonical wire formats remove that ambiguity.

Major section

Decode Bytes by Contract (continued)

It should apply the documented masks, test each flag, and leave reserved bits untouched so future firmware can add meanings without breaking old readers.

Both results are valid integers, which is why an incorrect decoder may fail silently rather than crash.
This supports the bitwise chapter's contract argument: multi-byte fields need an explicit byte order beside width, signedness, scale, and version before shifts or masks can produce trustworthy values.
A mask selects bitsAND with a mask reads or extracts; OR sets; AND with an inverted mask clears; XOR toggles.

Major section

Under the Hood: Packed Layouts, the Pipeline, and Validation

A reliable decoder follows the same path every time, and a packed byte is only useful when its layout is documented and tested.

Packed Status BytesPacking flags into one byte is common when every byte must be justified.
The humidity sample is ready for use.
The next uplink can be attempted.

The decode pipeline: load, order, mask and shift, scale, then validate.

Major section

Under the Hood: Packed Layouts, the Pipeline, and Validation (continued)

The measured supply is below the warning threshold.

Include a normal example and each boundary value; test invalid and reserved-bit states.
Common PitfallsSending raw multi-byte memory: Memory layout can differ from the wire format; assemble values explicitly before crossing a boundary.
In production, bitwise work is precise engineering.

Major section

Checkpoint: Summary

Endianness defines how a multi-byte value is assembled; resolve it before reading any number.

Masks select the bits that carry meaning: AND tests or extracts, OR sets, AND with an inverted mask clears, and XOR toggles.
Packed payloads need documentation, a worked example, and decoder tests kept with the source.
Validation catches wrong byte order, wrong field width, reserved-bit violations, and impossible states.

Deck summary

Key takeaways

The compact byte hides several settings that must be kept separate.

You will decode multi-byte fields, extract packed flags, and update selected register bits without disturbing their neighbours.
A payload may carry a two-byte temperature, a four-byte timestamp, and a one-byte status register.
The core rule is precise: bits are numbered positions inside a byte, and a mask selects the positions you care about.
Big-endian places the most significant byte first, so hexadecimal 12 34 becomes $18\times256+52=4660$ .

Retrieval practice

Recall check 1 of 3

Physics Phoebe says: answer from memory, then check your reasoning.

Q1A device packs eight boolean status flags into one byte. What should firmware use to test one flag without changing the others?

AConvert the byte to decimal and guess from the number's size

BOverwrite the whole byte with the flag value

CSend eight separate text strings for every status update

DMask the target bit before interpreting it.

Show answer

Answer: D ANDing with a mask reads the selected bit while leaving the unrelated flags untouched.

Retrieval practice

Recall check 2 of 3

Physics Phoebe says: answer from memory, then check your reasoning.

Q2A decoder accepts a packed status field. Which test set best checks that its layout will survive a firmware change?

ATest normal values repeatedly against the current firmware.

BTest a packet from the previous firmware version using the current decoder.

CTest the decoded display at several routine operating points.

DNormal values, both limits, and a case with a reserved bit set.

Show answer

Answer: D The limits and reserved state reveal parsing drift that a normal sample can hide.

Q3A protocol says a 16-bit field is big-endian. The received bytes are 01 90. What raw integer should the decoder assemble before scaling?

A0x9001

B0x0109

C0x1900

D0x0190

Show answer

Answer: D Big-endian places the most significant byte first, so 01 then 90 gives 0x0190.

Retrieval practice

Recall check 3 of 3

Physics Phoebe says: answer from memory, then check your reasoning.

Q4Firmware must enable only bit 2 of a shared control register without disturbing the other settings. Which operation is correct?

Areg = (1u << 2), because direct assignment sets only the target bit

Breg &= (1u << 2), because AND selects the target bit with a mask

Creg |= (1u << 2), because OR sets bit 2 while preserving neighbors

Dreg ^= (1u << 2), because XOR changes the selected bit while preserving neighbors

Show answer

Answer: C OR with a single-bit mask sets bit 2 while every other bit keeps its current value.

Print reference

Answers

Answer key.

D · ANDing with a mask reads the selected bit while leaving the unrelated flags untouched.
D · The limits and reserved state reveal parsing drift that a normal sample can hide.
D · Big-endian places the most significant byte first, so 01 then 90 gives 0x0190.
C · OR with a single-bit mask sets bit 2 while every other bit keeps its current value.