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Code: Registers and Busses
Code ch. 22 — registers as fast on-chip storage, the data/address/control busses that move bytes around, three-state buffers, and the fetch-execute cycle.
- Updated
- Reading time
- 3 min read
- Detail
- Code (2nd ed.)
- Detail
- Ch. 22
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In one line: registers are the CPU’s handful of ultra-fast storage boxes, and busses are the shared wires that shuttle bytes between them, the ALU and memory.
Companion to: Charles Petzold, Code (2nd edition), chapter 22.
Registers
Small stores built from flip-flops, right next to the ALU — the fastest memory there is.
| Register | Role |
|---|---|
| Accumulator | Holds the ALU’s working value |
| Program counter (PC) | Address of the next instruction |
| Instruction register | The instruction currently being decoded |
| Address register | The memory address being accessed |
| General-purpose registers | Scratch space (x86: EAX/RAX, EBX/RBX…) |
| Flags register | The status flags from chapter 21 |
Busses
A bus is a set of shared wires many components connect to.
| Bus | Carries | Width sets |
|---|---|---|
| Data bus | The bytes being moved | How many bits move at once (8, 16, 32, 64) |
| Address bus | Which memory location | How much memory can be addressed (2ⁿ) |
| Control bus | Signals like read, write, clock | — |
Three-state buffers are the trick that makes a shared bus work: each output can be 0, 1, or disconnected (high-impedance). Only the one device that’s enabled drives the bus at any moment; everything else stays disconnected.
The fetch-execute cycle
| Step | What happens |
|---|---|
| Fetch | Copy the instruction at the PC’s address into the instruction register |
| Decode | Work out the opcode and operands |
| Execute | Do it — ALU operation, memory access or a jump |
| Update PC | Advance to the next instruction (or jump elsewhere) |
This loop, billions of times a second, is all a CPU does.
Try it
Get-CimInstance Win32_Processor |
Select-Object Name, AddressWidth, DataWidth, NumberOfCores, MaxClockSpeedlscpu | grep -E 'Architecture|CPU\(s\)|MHz|cache'🔐 Security and IT connections
- 32-bit vs 64-bit is an address-bus story. A 32-bit address bus caps memory at 4 GB; 64-bit lifts it enormously. It’s why old software and drivers matter when you check whether a system is
AddressWidth32 or 64. - Registers hold live secrets. A debugger or exploit that reads register and stack state can lift keys and return addresses. Stack canaries and ASLR make that harder.
- The PC (program counter) is the ultimate target. Control-flow-hijacking attacks (buffer overflows, ROP) all aim to load an attacker-chosen address into it.
Practice drills
1. What does the program counter hold?
The memory address of the next instruction to fetch.
2. Why can several devices share one data bus without clashing?
Three-state buffers let all but the active device disconnect (high-impedance), so only one drives the wires at a time.
3. Which bus's width decides how much RAM a CPU can address?
The address bus — n lines address 2ⁿ locations.
Key takeaways
- Registers are the fastest storage, holding the accumulator, program counter, instruction and flags.
- Data, address and control busses move everything; three-state buffers share the wires safely.
- Fetch, decode, execute, repeat — the whole job of a CPU.