diff --git a/src/mame/drivers/unk6502_st2xxx.cpp b/src/mame/drivers/unk6502_st2xxx.cpp
index e64a255872f..14362245560 100644
--- a/src/mame/drivers/unk6502_st2xxx.cpp
+++ b/src/mame/drivers/unk6502_st2xxx.cpp
@@ -9,10 +9,14 @@
 // these games were ported to unSP hardware at some point, see generaplus_gpl_unknown.cpp
 
 // BIOS calls are made very frequently to the undumped firmware.
-// The most common call ($6058 in bbl380, $6062 in ragc153 & dphh8630) seems to involve downloading a snippet of code from Flash and executing it from RAM.
+// The most common call ($6058 in bbl380, $6062 in ragc153 & dphh8630) seems to involve downloading a snippet of code from Flash and executing it from RAM at $0300.
 // A variant of this call ($60d2 in bbl380, $60e3 in ragc153 & dphh8630) is invoked with jsr.
 // For these calls, a 24-bit starting address is specified in $82:$81:$80, and the length in bytes is twice the number specified in $84:$83.
 // ST2205U cannot execute code directly from Flash, but has a built-in DMA-compatible NAND interface on Port F ($05).
+// The XOR used on ragc153 & dphh8630 is likely performed by the DMA controller.
+// $6003 performs a table lookup, depositing a sequence of data at $008e.
+// $6000 generates some duration of delay based on the X register.
+// One other BIOS call ($6975 in bbl380, $69d2 in ragc153) has an unknown purpose.
 
 #include "emu.h"