Arrays: Packed, Dynamic, Queues & Associative · SystemVerilog

Part 1 of 9Language FoundationsIntermediate

4topics
4lessons

SystemVerilog has four array families. Packed arrays are single bit vectors, fixed unpacked arrays are collections of elements, dynamic arrays and queues are sized at runtime, and associative arrays are sparse maps keyed by value.

Five shapes, one question each: is the size fixed, does it grow, is the index sparse?

Family	Declaration	Size	Synthesizable
Packed	`logic [7:0] bus`	Fixed at compile time	Yes
Unpacked fixed	`logic [7:0] mem [0:255]`	Fixed at compile time	Yes
Dynamic	`int data[]`	`new[n]` at runtime	No, testbench only
Queue	`int q[$]`	Grows per push	No, testbench only
Associative	`byte mem [longint]`	One entry per written key	No, testbench only

Lesson	Path	You will build…
Packed vs unpacked	Required	Layout, slicing and copy rules that infer RAM instead of flops.
Dynamic arrays and queues	Required	Runtime sizing plus an in-order checker built on a queue.
Associative arrays	Required	A sparse memory and an out-of-order checker keyed by ID.
Array methods and selection	Recommended	Reductions, finders, sorting, and picking the right container. Recommended because scoreboards use `find`, `sort` and `unique`, but you can look the methods up as you need them.

Path: Required is the core first pass for this part's goal, a self-checking testbench; Recommended is what most learners should read; Optional is reference, specialist or revision material to return to later.

Check yourself A testbench must model a 64-bit address space but touches a few hundred locations. Which family fits, and why not a fixed array?

An associative array keyed by address. It allocates storage only for written keys. A fixed array of 2^64 bytes is impossible to allocate.

Up nextPacked vs Unpacked Arrays

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Family

Declaration

Size

Synthesizable

Packed

logic [7:0] bus

Fixed at compile time

Yes

Unpacked fixed

logic [7:0] mem [0:255]

Fixed at compile time

Yes

Dynamic

int data[]

new[n] at runtime

No, testbench only

Queue

int q[$]

Grows per push

No, testbench only

Associative

byte mem [longint]

One entry per written key

No, testbench only

Lesson

Path

You will build…

Packed vs unpacked

Required

Layout, slicing and copy rules that infer RAM instead of flops.

Dynamic arrays and queues

Required

Runtime sizing plus an in-order checker built on a queue.

Associative arrays

Required

A sparse memory and an out-of-order checker keyed by ID.

Array methods and selection

Recommended

Reductions, finders, sorting, and picking the right container. Recommended because scoreboards use find, sort and unique, but you can look the methods up as you need them.