Just take a look at TX19 firmware of 550D , lots of buttons found in RAM :
RAM:FFFFCC3C LockSW: .byte 1 # DATA XREF: ROM:000213BCo
RAM:FFFFCC3C # 0 - Lock(Off)
RAM:FFFFCC3D CardCover: .byte 1 # DATA XREF: sub_4BB14+B2r
RAM:FFFFCC3D # 0 - open
RAM:FFFFCC3E BatCover: .byte 1 # DATA XREF: sub_4BB14+E4r
RAM:FFFFCC3E # 0 - Open
RAM:FFFFCC40 SDDetectSw: .byte 0 # DATA XREF: sub_4BB14+116r
RAM:FFFFCC43 Sw1: .byte 1 # DATA XREF: sub_4BB14+148r
RAM:FFFFCC43 # 0 - on
RAM:FFFFCC44 Sw2: .byte 1 # DATA XREF: sub_4BB14+17Ar
RAM:FFFFCC44 # sub_6FED0+52r ...
RAM:FFFFCC45 AELockButton: .byte 1 # DATA XREF: sub_4BB14+1ACr
RAM:FFFFCC45 # 0 - on
RAM:FFFFCC48 SpdnButton: .byte 1 # DATA XREF: sub_4BB14+1DEr
RAM:FFFFCC48 # 0 - on
RAM:FFFFCC49 StroboPopUpButton:.byte 1 # DATA XREF: sub_4BB14+210r
RAM:FFFFCC49 # sub_6FED0+66r ...
RAM:FFFFCC4A StroboPopEndSw: .byte 0 # DATA XREF: sub_4BB14+242r
RAM:FFFFCC4E AFFrameSelectButton:.byte 1 # DATA XREF: sub_4BB14+274r
RAM:FFFFCC54 ISOButton: .byte 1 # DATA XREF: sub_4BB14+2A6r
RAM:FFFFCC57 Av_Button: .byte 1 # DATA XREF: sub_4BB14+2D8r
RAM:FFFFCC59 SetButton: .byte 1 # DATA XREF: sub_4BB14+30Ar
RAM:FFFFCC5A MenuButton: .byte 1 # DATA XREF: sub_4BB14+33Cr
RAM:FFFFCC5B PlayButton: .byte 1 # DATA XREF: sub_4BB14+36Er
RAM:FFFFCC5C DisplayButton: .byte 1 # DATA XREF: sub_4BB14+3D4r
RAM:FFFFCC5D EraseButton: .byte 1 # DATA XREF: sub_4BB14+3A0r
RAM:FFFFCC61 EasyDirect_QuickSetting:.byte 1 # DATA XREF: sub_4BB14+408r
RAM:FFFFCC63 CrossUp: .byte 1 # DATA XREF: sub_4BB14+470r
RAM:FFFFCC64 CrossDown: .byte 1 # DATA XREF: sub_4BB14+4A4r
RAM:FFFFCC65 CrossRight: .byte 1 # DATA XREF: sub_4BB14+4D8r
RAM:FFFFCC66 CrossLeft: .byte 1 # DATA XREF: sub_4BB14+50Cr
RAM:FFFFCC68 RECStartButton: .byte 1 # DATA XREF: sub_4BB14+43Cr
RAM:FFFFCC69 ModeDial: .byte 2 # DATA XREF: sub_4BB14+538r
