Return Stack

IOP-16 Stack

• Stack is used to keep track of return addresses for subroutines
  • Stores to stack on JSR instruction
  • Loads from stack with RTS instruction
• Last-In, First-Out (LIFO)
  • VHDL code
• 12-bit width

Configuring Depth

• Configurable stack depth
  • 0 = no stack
    • No JSRs or RTSs in code
    • Can do JMPs
  • 1 deep = Single deep calls
  • Optionally deeper as build option (uses more registers)
• Set STACK_DEPTH_PASS to the max nesting depth of the subroutines in the top file
• The LIFO capacity is equal to 2 ^ STACK_DEPTH_PASS

IOP16: ENTITY work.cpu_001
-- Need to pass down instruction RAM and stack sizes
	generic map 	( 
		INST_ROM_SIZE_PASS => 512, -- Small code size since program is "simple"
		STACK_DEPTH_PASS   => 4	   -- Subroutine depth (not nested)
	)

Resources

• STACK_DEPTH_PASS = 3
  • ALMs: 33
  • Registers: 86
• STACK_DEPTH_PASS = 4
  • ALMs: 54
  • Registers: 158