The BBC and Master Computer Public Domain Library

9883 Allocate space for new variable

Submitted by Steve Fewell

Routine:allocspace
Name: Allocate space for new variabler
Starting Address: &9983
Entry criteria: Y contains the length of the variable name (which has
just been copied to the VARTOP location) + 1.
X contains the number of bytes required for the variable parameter block.
Exit: The Variable parameter block is complete, and VARTOP has been updated
to point to the next free location.

Description:

Place a zero at the end of the variable name (to mark the end of the name).

Zero X number of bytes following the variable name location (starting at VARTOP + Y + 1).
This will clear the new variable parameter block.

Next, we need to find out if there is enough space for the variable name length (Y) plus allocation bytes (X) on the
BASIC variable stack (pointed to by VARTOP). This is done as follows.

[&988B] Calculate new VARTOP (&02-&03) location (which will point to the next free variable location (i.e. the
location after the variable name and parameter block info which has been stored for the current
variable)). But, don't update VARTOP with the new address yet.

If the new VARTOP location would be >= the BASIC Stack pointer, then we have run into the stack,
so store 0 for the Next variable MSB address field of the previous variable's parameter block (&3A,1),
to set that variable as being the last variable in the list, and generate a 'No Room' error.

Otherwise Store the updated VARTOP position back in &02-&03 and exit.

Disassembly for the Allocate space for new variable routine