Here is another G81 drilling cycle programming example which illustrates the use of repeat drilling in G91 Incremental Programming Mode.
Other CNC program example is here G81 Drilling Cycle – Repeat Drilling in G91 Incremental Mode Example Code
Repeat Drilling with G81 Drilling Cycle
O1000 T1 M6 G00 G90 G40 G21 G17 G94 G54 X20 Y10 S1000 M03 G43 H1 Z100 Z3 G81 G99 R3 Z-20 F350 M08 G91 X10 Y10 K4 G80 G00 G90 Z100 M30