Manipulation of activation records in Apoo

This is an extension of the Apoo vpu for dealing with activation records. It is available as version 3.0.

There are two programmable registers to address the system stack: stack register and frame register. They correspond to the last two registers of a Apoo vpu configuration, Rn-1 and Rn-2, but are aliased to rs and rf, respectively. The stack register rs contains the address of the last stack memory cell (or -1 if no static memory is allocated). The instructions jsr, rtn, push and pop manipulates the stack in the usual way. Besides that, the contents of the stack register can be manipulated as any other register.

The frame register can be used for the implementation of local information (on the system stack). It contents should be the first stack address of the current activation record. Like the stack register it can be manipulated as any other register, but it is also used in two special instructions: storeo and loado.

storeo Ri Num

stores the contents of register Ri at memory address (rf) + Num, where Num is an integer.

loadeo Num Ri

loads the contents of memory address (rf) + Num into register Ri, where Num is an integer.

In both instructions, if Num is non negative it should correspond to local memory and if it is negative, possibly corresponds to arguments of a subroutine call.

Here is an example of the use of these registers, for the implementation of subroutines with arguments (passed on the stack) and local memory.

n:	const 5
	loadn 4 r1
      # an argument
	push r1
	zero r1
	jsr test
	pop r1
	halt
test:	push rf 
	#saves the current frame pointer
	#current frame pointer
	storer rs rf 
	loadn  6 r2
	# reserves some local space
	add    r2 rs 
	# gets the argument  
        loado -2 r1 
      # only testing the rs
	push r1    
	pop r1
	#stores contents of r1 at rf+1
	storeo r1 1
	#loads the same value into r3
	loado 1 r3  
	# restores stack before return 
	sub rs r2   
	storer r2 rs
	# restores frame before return
	pop rf      
	rtn

A more complete example can be found here.