Difference between revisions of "Return Stack"

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= IOP-16 Stack =
 
= IOP-16 Stack =
  
* Stack stores the return address
+
* Stack is used to keep track of return addresses for subroutines
 
** Stores to stack on JSR instruction
 
** Stores to stack on JSR instruction
 
** Loads from stack with RTS instruction
 
** Loads from stack with RTS instruction

Revision as of 14:49, 19 April 2022

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)
  • Sizes
    • 0 (none)
      • No JSRs or RTSs in code
      • Can do JMPs
    • 1 deep (does not need SRAM)
      • 12-bit register
      • Single deep calls
    • Optionally deeper as build option (uses more registers)

Configuring Depth

  • 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 = 4
    • ALMs: 54
    • Registers: 158