COMMON		8F	??
CPMSYS		8E	00
HBIOS		8D	??
TPA		8C	01
BUFS		8B	02

8D:7000 -> 8E:0300


00 -> 8E
01 -> 8C
02 -> 8B
03 -> 8A
...

if bnk = 0, then hbbnk = 8EH (BID_USR)
else hbbnk = 8DH  (BID_BIOS) - bnk

	or	a
	jr	z,bank0
	neg			; 2 -> -2
	add	8DH		; 8D - 2 = 8B
	jp	HBX_SETBNK
bank0:
	ld	a,(8EH)
	jp	HBX_SETBNK
	ret