Elmo HARmonica, HARSFEN0602 software manual Data types, Usrsubj

HARSFEN0602ElmoHARmonicaSoftwareManual PRELIMINARYDRAFT

[x,y] = f (a,b,c,d) ;

Assume that BP before function call is 12 and SP is 15 (just number not better and not worse than every other).

Before the opcode USRSUBJ is executed the stack contains:

2.entries for actual output arguments (according to the function call, not prototype)

3.entries for input arguments (a number of input arguments during function call must be the

same as in the prototype)

In our example the stack pointer before USRSUBJ is equal to 21: is was 15 + 2 of actual output arguments + 4 of input arguments.

Now the opcode USRSUBJ is executing. During USRSUBJ execution stack is increased by 5 service entries (from SP = 21 to SP = 25) and number of maximal output arguments according to the function definition. The meaning of these service entries is described in the table below. The number of maximal possible output arguments in our example is 3: x, y and z, from SP = 26 to SP = 28).

When 5 service entries has set to the stack a new value of BP is assigned (in our example 26), while the previous value (12) is saved for later restoration.

This table below shows the contents of the stack for our example

SP	Meaning	Value	Remark
12			BP before function call
…
15	Place for actual output argument y	Junk	SP before function call
16	Place for actual output argument x	Junk
17	Input argument	Value of a
18	Input argument	Value of b
19	Input argument	Value of c
20	Input argument	Value of d
21	Save BP current	12	USRSUBJ start
22	Return address	Value of return	One of operands of
		address	USRSUBJ
23	Index of the function in the Function	Function index	One of operands of
	Symbol Table		USRSUBJ
24	Number of input arguments	4	Data from the Function
			Symbol Table
25	Number of actual output arguments	2	One of operands of
			USRSUBJ
26	Place for output argument x	Junk	New value of BP
27	Place for output argument y	Junk
28	Place for output argument z	Junk
29			USRSUBJ end.
…

After the opcode USRSUBJ ends SP is 29 and BP is 26.

During USRSUBJ the jump to the beginning of the function is executed (PC is assigned to the relevant location in the Code Segment). Further filling of the stack is fulfilled according to the executable code of the function.

Every function must end with return (the opcode USRSUBRT).

During USRSUBRT the stack returns to the initial state before function call, but not exactly.

USRSUBRT copies the actual output arguments to preliminary allocated place in the inverted order. In our example z finally won’t be assigned, x will be copied from SP = 26 to SP = 16 and y will be copied from SP = 27 to SP = 15.

BP is restored to the previous value (12). Service entries, input and output arguments are removed and finally SP is 17.

72

7.4Data types