📚 Packed vs Unpacked Arrays

Basically packed arrays are the size of each element and unpacked arrays is how many of these elements there are

✅ Packed Array

• Declared before the variable name: [N:0] var
• Represents a bit vector — tightly packed bits
• Treated as a single unit in memory
• Can be sliced, indexed, and used in arithmetic

reg [7:0] byte;       // Packed: 8 bits in one unit
byte[3] = 1'b1;       // ✅ Bit-level access

• Used for bit-level operations, arithmetic, protocols
• Example: reg [31:0] instruction;

✅ Unpacked Array

• Declared after the variable name: var [M:0]
• Represents an array of elements
• Each element is a separate packed unit (or scalar)
• Indexed like a traditional array

reg [7:0] mem [255:0];  // Unpacked: 256 elements, each 8 bits
mem[42] = 8'b10101010;  // ✅ Element-level access

• Used for storage, indexing, looping
• Example: reg [7:0] memory [0:255];

🧩 Example

reg [7:0] mem [255:0];

So:

• mem[42] → 8-bit packed word
• mem[42][3] → bit 3 of that word