The
6502 microprocessor was extremely
register poor. However, the first
page (a page was
256 bytes, so this was
addresses &00-&FF in the
BBC's
hex notation), called
zero page, was often used instead of a larger
register file.
Storing data in zero page had several advantages. First, many instructions had special-case opcodes when refering to zero page. These were naturally 2-byte rather than 3-byte, so you saved a byte. And they were generally a cycle faster, too! And some instructions could only be used with a zero page operand.
For these reasons, everybody on a 6502-based box wanted to use as much of zero page for themselves as possible. On the BBC micro, by the time the OS, language ROMs, DFS and others were done, applications were left with just the range &70-&8F to use for themselves...