HP 4 FAMILY - Manual (Page 131)

The command byte itself has three parts:
Control Bit Offset Count Replacement Count
Control Bit. Determines whether the replacement data
is compressed, and also the bit boundaries of the
command byte’s other two fields.
Offset Count. The left offset (number of bytes) the
replacement data is offset from the current byte position
in the seed row.
Replacement Count. The number of consecutive bytes
to be replaced. One more byte than the replacement
count is replaced (for example, 6 bytes are replaced by a
replacement count of 5).
Like compression method 3, the “current” byte follows the
last replacement byte; at the beginning of a row, the current
byte immediately follows left raster margin. An offset of 0
indicates the current byte; an offset of 1 indicates the byte
following the current byte.
The size of the offset count and replacement count fields
depends on the value of the control bit.
CONTROL BIT = 0
7 6 3 2 0
Control Bit = 0 Offset Count Replacement Count
If the control bit is 0, the replacement data is
uncompressed. If the control bit is 0, bits 0-2 indicate the
replacement count and bits 3-6 indicate the offset count.
If the offset count is 15, an additional offset count byte
follows and is added to the total offset count. If the offset
count byte is 255, another offset count byte follows. The last
offset count byte is indicated by a value less than 255.
If the replacement count is 7, an additional replacement
count byte follows and is added to the total replacement
Printer-Specific Differences 2-95