The BBC and Master Computer Public Domain Library

99AE '!' and '?' address modifier operators

Submitted by Steve Fewell

Routine:addrmodify
Name: '!' and '?' address modifier operators
Starting Address: &99AE (for '!') or &99B0 with A=0 (for '?')
Entry criteria: &2A and &2B point to the contents of the variable.
The Text Pointer B points to the next character of the program line.
Exit:(&2A, &2B) contain the adjusted address (variable value + modifier value).
&2C contains the return type (of the value to obtain from the new address).

Description:

Store the return value type on the stack (0 for 1-byte Integer or 4 for 4-byte Integer).
Store back the offset in &1B, as we have correctly evaluated the variable address.
Load the variable value to obtain the base-memory address to modify.
If the variable is a string, then it is not a Memory address so generate a Type mismatch error.

Store the Integer base-address MSB (&2B) on the Stack.
Store the Integer base-address LSB (&2A) on the Stack.
Get the Integer value at PTR B. This provides the value to modify the base-address by.

Add the Integer value to the Base memory address (from the Stack). Now (&2A, &2B) contain
the modified address.
Exit with Carry clear and A = #&FF (meaning that the routine ended sucessfully).

Disassembly for the '!' and '?' address modifier operators routine