VHDL Programming

94 Chapter Four

TYPE instruction IS

RECORD

opcode : optype;

src : INTEGER;

dst : INTEGER;

END RECORD;

The first line declares the enumerated type optype, which is used as

one of the record field types. The second line starts the declaration of the

record. The record type declaration begins with the keyword RECORDand

ends with the clause END RECORD. All of the declarations between these

two keywords are field declarations for the record.

Each field of the record represents a unique storage area that can

be read from and assigned data of the appropriate type. This example

declares three fields:opcodeof typeoptype, andsrcanddstof type

INTEGER. Each field can be referenced by using the name of the record,

followed by a period and the field name. Following is an example of this

type of access:

PROCESS(X)

VARIABLE inst : instruction;

VARIABLE source, dest : INTEGER;

VARIABLE operator : optype;

BEGIN

source := inst.src; --Ok line 1

dest := inst.src; --Ok line 2

source := inst.opcode; --error line 3

operator := inst.opcode; --Ok line 4

inst.src := dest; --Ok line 5

inst.dst := dest; --Ok line 6

inst := (add, dest, 2); --Ok line 7

inst := (source); --error line 8

END PROCESS;

This example declares variable inst, which is of type instruction. Also,

variables matching the record field types are declared. Lines 1 and 2 show

fields of the record being assigned to local process variables. The assign-

ments are legal because the types match. Notice the period after the name

of the record to select the field.

Line 3 shows an illegal case. The type of field opcodedoes not match

the type of variable source. The compiler will flag this statement as a type

mismatch error. Line 4 shows the correct assignment occurring between

the field opcodeand a variable that matches its type.