HP 4 FAMILY - Manual (Page 132)

count. If the replacement count byte is 255, another
replacement count byte follows. The last replacement count
byte is indicated by a value less than 255. One more than
the total replacement byte count will be replaced.
CONTROL BIT = 1
7 6 5 4 0
Control Bit = 1 Offset Count Replacement Count
If the control bit is 1, the replacement data is run length
encoded. the bit boundaries are different than if the control
bit is 0: bits 5-6 contain the offset count, and bits 0-4
contain the replacement count. As when the control bit is 0,
optional offset bytes and replacement bytes may be added.
2-96 Printer-Specific Differences