80-Bus News

The workings of Nascom ROM BASIC Ver 4.7

PAGE 8

******** How variables and arrays are stored ********

Variables such as AB AB$ and FN AB are all stored in the simple variable area of memory. The start address of this area is held in PROGND and the end address is held in VAREND.

If AB=10 and AB$="TEXT” and FN AB(XY) had been defined then the memory would look like this

42 41	Name of AB in reverse (42 41 = “B” “A")
00 00 20 84	Floating point value for 10

C2 41	Name for AB$ (C2 is “B” with bit 7 set)
04	Length of string (4 characters)
??	This byte unused for string
LL HH	Address where “TEXT” is to be found

42 C1	Name of FN AB (C1 is “A” with bit 7 set)
LL HH	Address of function (After “=")
59 58	Argument name in reverse (59 58 = “Y” “X")

Arrays such as AB(1,3) and AB$(3,1) are stored in the array area of memory. The start address of this area is held in VAREND and the end address is held in ARREND.

If DIM AB(1,3),AB$(3,1) had been entered then the memory would look like this:–

42 41	Name of array AB in reverse
25 00	Bytes used for array (0025 = 37)
02	2 dimensions
04 00	Size of second dimension including zero element
02 00	Size of first dimension including zero element
00 00 00 00	AB(0,0)
00 00 00 00	AB(1,0)
00 00 00 00	AB(0,1)
00 00 00 00	AB(1,1)
00 00 00 00	AB(0,2)
00 00 00 00	AB(1,2)
00 00 00 00	AB(0,3)
00 00 00 00	AB(1,3)

C2 41	Name of array AB$ in reverse
25 00	Bytes used for array (0025 = 37)
02	2 dimensions
02 00	Size of second dimension including zero element
04 00	Size of first dimension including zero element
00 00 00 00	AB$(0,0)
00 00 00 00	AB$(1,0)
00 00 00 00	AB$(2,0)
00 00 00 00	AB$(3,0)
00 00 00 00	AB$(0,1)
00 00 00 00	AB$(1,1)
00 00 00 00	AB$(2,1)
00 00 00 00	AB$(3,1)

The workings of Nascom ROM BASIC Ver 4.7

PAGE 9

******** Usage of the stack for GOSUB/RETURN and FOR/NEXT ********

GOSUB and RETURN usage of the stack

When a GOSUB is executed the address of where to RETURN to and the number of the line to RETURN to are PUSHed on the stack as follows:–

HIGH MEMORY:	XX XX	Address of where to RETURN to
	XX XX	Number of line to RETURN to
STACK POINTER:	8C	GOSUB token as marker

This GOSUB block remains on the stack until a RETURN is executed at which point BASIC looks back through the stack until it finds a GOSUB block and then sets the stack there and then POPs the line number and the address of the statement after the GOSUB and continues execution.

The fact that the stack is set to this GOSUB block kills all active FOR loops which were set up inside the subroutine.

FOR and NEXT usage of the stack

When a FOR is executed the address of the first statement in the loop, the line number of the loop statement, the TO value, the STEP value and the sign of the STEP are all PUSHed onto the stack as follows:–

HIGH MEMORY:	XX XX	Address of first statement in loop
	XX XX	Line number of loop statement
	XX XX XX XX	TO value in floating point
	XX XX XX XX	STEP value in floating point
	XX	Sign of STEP
	XX XX	Address of index variable
STACK POINTER:	81	FOR token as marker

This FOR block remains on the stack until a matching NEXT is executed. When next is executed BASIC looks back through the stack to find the matching FOR block. If it is found then the STEP value is added to the value of the index variable and the result is compared with the TO value. With the use of the TO value and the sign of the step BASIC knows if the loop has been completed or not. If it has not been completed then the FOR block remains on the stack until the loop has been completed. The stack is set to point to this FOR block and effectively kills all FORs nested within this loop. When the loop is completed the FOR block is removed from the stack and execution continues from after the NEXT instruction.

If the FOR block cannot be found then a “?NF Error” occurs.

May–June 1983 · Volume 2 · Issue 3

GOSUB and RETURN usage of the stack

FOR and NEXT usage of the stack