boot0 uses EXI to
capture events coming from surface mounted components (SMC). If a special button combo is being held (additional hardware may be required, like in the case of kiosk units), boot0 will attempt to load a recovery signed boot1 image from a SD card.
// Send debug mark
u32 exi0_reply = *(u32 *)exi0_out_buf;
SD card button combo was pressed
if ((exi0_reply << 0x14) < 0)
load_sd = true;
No SD card combo, we're done