Whenever using IY, FD is the prefix. In cases of using it as an address, you also need an offset byte. IY and IX are the z80 Index Registers and can be used in most places that HL can be used. (This is where knowing hex makes things a lot easier).
ld hl,0 ;210000
ld ix,0 ;DD210000
ld iy,0 ;FD210000
add hl,de ;19
add ix,de ;DD19
add iy,de ;FD19
;Using as an address
ld a,(hl) ;7E reads the byte at (hl) into a
ld a,(ix+3) ;DD7E03 reads the byte at (ix+3) into a
ld a,(iy+3) ;FD7E03
set 0,(hl) ;CBC6 sets bit 0 of the byte at (HL)
set 0,(ix+0) ;DDCB00C6 sets bit 0 of the byte at (ix+0)
set 0,(iy+0) ;FDCB00C6 sets bit 0 of the byte at (ix+0)
Now, in the case of the instruction you gave, it is actually undocumented, but pretty neat. I have known the instruction as set imm3,(iy+offs8),reg8 which basically means you can set bits 0 to 7 at (iy+signed_offset), but then the result of the byte at (iy+unsigned_offset) gets copied to the 8-bit register. This also works with BIT, RES, and any of the 2-byte shift/rotate commands, using IX or IY.
EDIT: Also, the hex code would basically be whatever set imm8,reg8 would be, with wither FD or DD before it, and the offset after the CB. So set 0,(ix+0),c would be DDCB00C1.
