Sequential Statements

1
Sequential Statements

• Variable Assignment
• For example, if a variable r is a record with two
  fields a and b, then they could be exchanged by
  writing
• (a gt r.b, b gt r.a) r
• If Statement
• if_statement if condition then sequence_of_st
  atements elsif condition then sequence_of_state
  ments else sequence_of_statements end if

2
If Statement (con)

• Example 1
• if en1 then stored_value data_in
• end if
• Example 2
• if sel0 then result lt input_0 -- executed
  if sel 0
• else
• result lt input_1 -- executed if sel / 0
• end if

3
If Statement (con)

• Example 3
• if opcode halt_opcode then PC
  effective_address -- all three statements are
• executing false -- are executed one after
• halt_indicator true -- another
• end if
• Example 4 (nested)
• if phase wash then
• if cycle_select delicate_cycle then
• agitator_speed lt slow
• else agitator_speed lt slow
• end if
• agitator_on lt true
• end if

4
If Statement (con)

• There are also attributes of signals (S). The
  most common type use with if statement is
• Sevent True if there is an event on S in the
  current simulation cycle, false otherwise
• Example 5
• if (clkevent and clk1) then -- active when
  rising edge
• .
• end if
• if (clkevent and clk0) then -- active when
  falling edge
• .
• end if

5
Case Statement

• Case Statement
• Some examples of case statements
• case element_colour of when red gt statements
  for red when green blue gt statements for
  green or blue when orange to turquoise
  gt statements for these coloursend case
• case opcode of when X"00" gt perform_add when
  X"01" gt perform_subtract when others gt
  signal_illegal_opcodeend case

6
Case Statement (con)

• Example
• type opcodes is
• (nop, add, subtract, load, store, jump,
  jumpsub, branch, halt)
• case opcode is
• when load add subtract gt
• operand memory_operand
• when store jump jumpsub branch gt
• operand address_operand
• when others gt -- for last alternatives
• operand 0
• end case

7
Case Statement (con)

• The previous case statement could be rewritten as
• case opcode is
• when add to load gt
• operand memory_operand
• when branch downto store gt
• operand address_operand
• when others gt
• operand 0
• end case

8
Case Statement (con)

• Another way is to use range constraint
• subtype control_transfer_opcodes is opcodes
  range jump to branch
• case opcode is
• when control_transfer_opcodes store gt
• operand address_operand

9
Case Statement (con)

• Case statement must all be written using locally
  static value
• Illegal example
• variable N integer 1
• case expression is
• when N N1gt -- illegal since N might
  change during
• when N2 to N5 gt -- the course of
  execution, these choices
• when others gt -- are not locally static
• end case
• Legal example
• constant C integer 1
• case expression is
• when C C1gt
• when C2 to C5 gt
• when others gt
• end case

10
Loop Statement

• Loop Statements
• An example of such a basic loop is
• loop do_somethingend loop
• An example of using while loop statment
• while index lt length and str(index) / ' '
  loop index index 1end loop
• An example of using For Loop statment
• for item in 1 to last_item loop table(item)
  0end loop

11
Loop Statements Con..

• for i in 1 to max_str_len loop a(i)
  buf(i) exit when buf(i) NULend loop
• outer_loop loop inner_loop
  loop do_something next outer_loop when temp
  0 do_something_else end loop
  inner_loopend loop outer_loop

12
Sequential Statements

• Null Statement
• The null statement has no effect. It may be used
  to explicitly show that no action is required in
  certain cases. example
• case controller_command is when forward gt
  engage_motor_forward when reverse gt
  engage_motor_reverse when idle gt nullend
  case

13
Sequential Statements

• Assertions.
• An assertion statement is used to verify a
  specified condition and to report if the
  condition is violated.
• The syntax is
• assertion_statement assert condition
  report expression severity expression