The BBC and Master Computer Public Domain Library

Check for Stack clash with Heap

Submitted by Steve Fewell

Routine: checkstackclash
Name: Check for Stack clash with Heap
Starting Address: &BD1E
Entry criteria: A = The BASIC Stack Pointer Low Byte, after the required number of bytes of storage space has been subtracted. C (Carry flag) = 0 if overflow occurred during the subtraction of the required number of bytes.

Description:
Store A back to the Stack Pointer Low Byte. Decrement &05 (The Stack Pointer High Byte), if necessary.

If the Stack Pointer High Byte is now less than the Heap high byte then there is no room in memory, so error (as the Stack is stored above the Heap).

If the Stack Pointer High Byte is equal to the Heap high Byte, then test the low bytes. If the Stack Pointer Low Byte is less than the Heap Low Byte then a "No Room" error is produced, otherwise the routine returns successfully.

Disassembly for the Check for Stack clash with Heap routine