HP 7700 Series User Manual page 30

Processing Commands
When a command is entered, the Clear_Output routine clears the
destination area. Then, the instructions from Process_Cmd to Cmd_A
determine whether the command was "A", "B", or an invalid command.
If the command input byte is "A" (ASCII 41 hex), execution is
transferred to the instructions at Cmd_A.
If the command input byte is "B" (ASCII 42 hex), execution is
transferred to the instructions at Cmd_B.
If the command input byte is neither "A" nor "B", an invalid command
has been entered, and execution is transferred to the instructions at
Cmd_I.
The instructions at Cmd_A, Cmd_B, and Cmd_I each load
accumulator A with the length of the message to be displayed and
index register X with the starting location of the appropriate message.
Then, execution transfers to Output which writes the appropriate
message to the destination location, Msg_Dest.
After the message is written, the program branches back to read the
next command.
The Destination Area
The "BUFFER" section declares memory storage for the command
input byte, the destination area, and the stack area.
2-6 Getting Started