RES - Z80 instruction reference

resReset bit 
Clears one bit of a register or memory byte to zero, leaving the other bits unchanged.
Used in 8-bit and 16-bit arithmetic, logic and bitsZ80 assembly
 
SZHP/VNC ––––––
Changes no flag
Forms  res ,    2 bytes · 8 cycles 
Resets bit b of r.
 res ,    2 bytes · 15 cycles 
Resets bit b of (HL).
 res ,    4 bytes · 23 cycles 
Resets bit b of the memory location pointed to by IX plus dd.
Undocumented 1 rowLeft out of Zilog's manual, but the Z80 runs them, and so does this emulator.
  res , ,    4 bytes · 23 cycles 
Resets bit b of the memory location pointed to by IX plus dd. The result is then stored in r.
Operands b A bit number from 0 to 7, where bit 0 is the least significant bit. 
r An 8-bit register: a, b, c, d, e, h or l. 
(hl) The byte in memory at the address stored in hl. More
(ix+dd) The byte in memory at ix + dd, where dd is a signed displacement from -128 to 127, for example (ix+5) or (ix-3). (iy+dd) works the same way. More
         .org 0x8000
        ld b, 3
        res 0, b
        halt