Selected assignment (with-select) in VHDL - Step-by-Step Execution

Concept Flow - Selected assignment (with-select)

Start

↓

Evaluate select expression

↓

Match expression to choice

↓

Choice 1?

Yes→Assign value 1 to target

↓

Choice 2?

Yes→Assign value 2 to target

↓

Others

Yes→Assign default value

↓

End

The selected assignment evaluates an expression and assigns a value based on matching choices, with a default if no match.

Execution Sample

VHDL

with sel select
  out <= "0001" when "00",
         "0010" when "01",
         "0100" when "10",
         "1000" when others;

Assigns a 4-bit output based on 2-bit select input using selected assignment.

Execution Table

Step	sel value	Matched choice	Assigned out value	Notes
1	"00"	"00"	"0001"	sel matches first choice
2	"01"	"01"	"0010"	sel matches second choice
3	"10"	"10"	"0100"	sel matches third choice
4	"11"	others	"1000"	sel matches default (others)
5	-	-	-	End of selected assignment execution

💡 All possible sel values matched; assignment complete.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
sel	undefined	"00"	"01"	"10"	"11"	varies per step
out	undefined	"0001"	"0010"	"0100"	"1000"	varies per step

Key Moments - 2 Insights

Why does the 'others' choice assign a value when sel is "11"?

Because 'others' covers all values not explicitly listed, so when sel is "11" it matches 'others' as shown in execution_table row 4.

What happens if sel matches multiple choices?

In VHDL selected assignment, sel matches only one choice; the matching choice is used. The execution_table shows unique matches per step.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'out' when sel is "10"?

A "0100"

B "1000"

C "0010"

D "0001"

💡 Hint

Check execution_table row 3 under 'Assigned out value'.

At which step does the 'others' choice get assigned?

A Step 1

B Step 3

C Step 4

D Step 2

💡 Hint

Look at execution_table row 4 under 'Matched choice'.

If sel is "01", what will 'out' be assigned according to the execution table?

A "0001"

B "0010"

C "0100"

D "1000"

💡 Hint

Refer to execution_table row 2 for sel "01".

Concept Snapshot

Selected assignment syntax:
with <expression> select
  <target> <= <value1> when <choice1>,
             <value2> when <choice2>,
             ...
             <default_value> when others;

Behavior:
Evaluates expression once,
assigns target based on matching choice,
uses 'others' as default fallback.

Full Transcript

This visual execution trace shows how a VHDL selected assignment works. The select expression 'sel' is checked against each choice. When a match is found, the corresponding value is assigned to 'out'. If no explicit match occurs, the 'others' choice assigns a default value. The execution table steps through different sel values and shows the assigned output. The variable tracker records how 'sel' and 'out' change at each step. Key moments clarify why 'others' is important and how matching is unique. The quiz tests understanding of matching and assignment. The snapshot summarizes syntax and behavior for quick reference.