*MIDI redirect could use sound buffer at &870-&87F as note buffer.
Sideways ROM redirect could use buffer 5,6 as intercept code.

0850 CMP #7
0852 BEQ romcall
0854 JMP 0
0857 LDA &F4
0859 PHA
085A LDA #0
085C JSR select
085F JSR redirect
0862 PLA
     .select
0863 STA &F4
0865 STA &FE30
0868 LDA #7
086A RTS
086B EQUB 0
086C EQUD 0
0870 note buffer