ALU and Execution Engines

ALU = Arithmetic Logic Unit

A fundamental CPU component that's been around since the 1940s. It's the part of a processor that does math and logic operations.

What ALU Handles

• Arithmetic: +, -, *, //, %
• Logic: & (AND), | (OR), ^ (XOR)
• Shifts: <<, >>
• Comparisons: <, ==

Every CPU has an ALU. Your laptop, phone, even a calculator - all have ALUs.

VALU = Vector ALU

The vector version of ALU - does the same operations but on 8 values simultaneously.

Typical VLIW Execution Units

Modern VLIW processors commonly include these types of execution units:

The exact number of slots varies by processor. For example:

• TI C6x DSPs: 8 functional units (2 multipliers, 6 ALU-like)
• Qualcomm Hexagon: 4 execution slots per cycle
• Intel Itanium: Up to 6 instructions per bundle

What "Slots per Cycle" Means

Each engine can execute multiple operations per cycle (up to its slot limit):

1
2
3
4
5
6
7
8

# This is ONE cycle - all 3 ALU operations happen simultaneously
{
    "alu": [
        ("+", 0, 1, 2),   # scratch[0] = scratch[1] + scratch[2]
        ("-", 3, 4, 5),   # scratch[3] = scratch[4] - scratch[5]
        ("*", 6, 7, 8),   # scratch[6] = scratch[7] * scratch[8]
    ]
}

The ALU engine has 12 slots, so you could pack up to 12 independent ALU operations in a single cycle.

Why Multiple Engines Matter

Different engines work in parallel. In one cycle you can:

• Do 12 ALU operations AND
• Do 6 VALU operations AND
• Do 2 loads AND
• Do 2 stores AND
• Do 1 flow operation

The combined throughput across all engines represents the theoretical maximum operations per cycle. Achieving this requires finding enough independent operations to fill all slots.

Real CPUs Use These Terms

• Intel/AMD x86: Has multiple ALUs per core
• GPUs (NVIDIA/AMD): Thousands of ALUs for parallel processing
• ARM: ALU is part of the core pipeline

The terminology in this challenge mirrors real processor architecture.