Return Stack

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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)
  • 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 - 1
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