C10 <Left Shoulder> <Right Shoulder> C11 C15 M1 M0 M7 Y M2 D11 M6 < Joystick C12 A B C13 M3 M4 M5 X C14 D6 D7 < Vol > Sel Start C9 C8 Letters state which GPIO button is connected to, number states which bit in that GPIO. All buttons are active low.
Other things: -- SD -- D2 = Lock status (1 = Locked, 0 = Unlocked or MMC card) I14 = Insertion status (1 = no SD inserted, 0 = SD inserted) L0 = 4-bit access  L1 = 4-bit access  L2 = 4-bit access  L3 = 4-bit access  L4 = Command L5 = Clock -- LCD -- B2 = CX25874 Sleep mode (1 = sleep, 0 = awake) B3 = CX25874 Reset mode (0 = reset, 1 = normal operation) B4 = VSync B5 = HSync B6 = DE (Data enable) B7 = CLKH (Pixel clock output) H1 = VGH H2 = Backlight (1 = on, 0 = off) H11 = Backlight (1 = on, 0 = off) (F200) -- NAND -- I0 = Flash enable I2 = Write protect J0 = Write enable J1 = Output (Read) enable J2 = Ready/nBusy J9 = Address Latch Enable J10 = Command Latch Enable O1 = Block size (0 = large block, 1 = small block) -- Audio -- L6 = Data out L7 = Reset L8 = Sync L9 = Data In L10 = Clock L11 = Headphone jack detect -- USB (Destination: NetChip 2272) -- D4 = Reset ? E0 - E15 = Data bus G1 - G5 = Address bus (with 16-bit data bus, this makes addresses 0x00 - 0x1F) I9 - I10 = Static memory chip select J11 = NPWE, Write strobe to USB chip J12 = NPOE, Read strobe to USB chip K1 = DVAL1, DMA Validate signal K5 = DREQ1, DMA Request signal -- UART -- D0 = UART0 transmit (GP2X->PC) D1 = UART0 receive (PC->GP2X) -- Misc -- H4 = Battery LED ( 0 = on, 1 = off ) (UART0 refers to one connected by gph provided serial/jtag board)
Don't know how to use this information? Take a look at the GPIO Programming Example