Concept Flow - Subprogram call (M98) and return (M99)
Start Main Program
↓
Encounter M98 Call
↓
Jump to Subprogram
↓
Execute Subprogram
↓
Encounter M99 Return
↓
Return to Main Program
↓
Continue Main Program
↓
End Program
The main program runs until it hits M98, which calls a subprogram. The subprogram runs until M99, which returns control back to the main program to continue.
Main program moves to X0 Y0, calls subprogram 100 with M98, then moves to X10 Y10. Subprogram 100 moves to X5 Y5 and returns with M99.
Execution Table
Step
Program Line
Action
Position (X,Y)
Program Pointer
1
N10
Rapid move to X0 Y0
(0,0)
Main N10
2
N20
Call subprogram 100 (M98)
(0,0)
Main N20 -> Subprogram O100 N100
3
N100
Linear move to X5 Y5
(5,5)
Subprogram N100
4
N110
Return from subprogram (M99)
(5,5)
Return to Main N30
5
N30
Linear move to X10 Y10
(10,10)
Main N30
6
N40
Program end (M30)
(10,10)
Main N40
7
-
Program stops
(10,10)
End
💡 Program ends after M30 at line N40
Variable Tracker
Variable
Start
After Step 1
After Step 2
After Step 3
After Step 4
After Step 5
After Step 6
Position (X,Y)
(unknown)
(0,0)
(0,0)
(5,5)
(5,5)
(10,10)
(10,10)
Program Pointer
Main N10
Main N20
Subprogram N100
Subprogram N110
Main N30
Main N40
End
Key Moments - 3 Insights
Why does the program jump to subprogram at M98 instead of continuing the main program?
At step 2 in the execution_table, M98 tells the machine to call the subprogram specified by P100, so the program pointer moves from main line N20 to subprogram line N100.
What happens when the program reaches M99 in the subprogram?
At step 4, M99 signals the end of the subprogram and returns control back to the main program at the line after the M98 call, which is N30.
Does the position change when returning from the subprogram?
No, as shown in the variable_tracker, the position remains at (5,5) after M99 before moving to the next main program line.
Visual Quiz - 3 Questions
Test your understanding
Look at the execution_table, what is the position after executing the subprogram line N100?
A(5,5)
B(0,0)
C(10,10)
D(unknown)
💡 Hint
Check the Position column at Step 3 in the execution_table.
At which step does the program return from the subprogram to the main program?
AStep 2
BStep 5
CStep 4
DStep 6
💡 Hint
Look for the action 'Return from subprogram (M99)' in the execution_table.
If M99 was missing in the subprogram, what would happen to the program pointer after step 3?
AIt would return to main program immediately
BIt would continue in the subprogram lines
CIt would stop the program
DIt would jump to a different subprogram
💡 Hint
M99 is needed to return; without it, the subprogram continues executing.
Concept Snapshot
M98 calls a subprogram specified by P-code.
M99 ends the subprogram and returns to the main program.
Main program pauses at M98 until subprogram finishes.
Position and state carry over between main and subprogram.
Use M30 to end the main program.
Full Transcript
This visual execution shows how a CNC program uses M98 to call a subprogram and M99 to return. The main program runs lines N10 and N20, then calls subprogram O100 at N20. The subprogram runs lines N100 and N110, moving the tool and then returning control with M99. After return, the main program continues at N30 and ends at N40 with M30. The position updates as moves happen and stays consistent across calls and returns. This helps beginners see how subprogram calls pause the main program and how returns resume it.
Practice
(1/5)
1. What does the M98 command do in CNC programming?
easy
A. Stops the machine immediately
B. Ends the current subprogram and returns to the main program
C. Starts the spindle rotation
D. Calls a subprogram by its number, optionally repeating it
Solution
Step 1: Understand the role of M98
The M98 command is used to call a subprogram by its number and can repeat it multiple times if specified.
Step 2: Differentiate from M99
M99 is used to end a subprogram and return to the main program, not to call it.
Final Answer:
Calls a subprogram by its number, optionally repeating it -> Option D
Quick Check:
M98 calls subprogram [OK]
Hint: M98 calls subprogram; M99 returns from it [OK]
Common Mistakes:
Confusing M98 with M99
Thinking M98 stops the machine
Assuming M98 starts spindle
2. Which of the following is the correct syntax to call subprogram number 100 with 3 repetitions?
easy
A. M100 P98 L3
B. M98 P100 L3
C. M98 L3 P100
D. M99 P100 L3
Solution
Step 1: Identify correct M98 syntax
The correct syntax to call a subprogram is M98 followed by P (program number) and optionally L (repeat count), so M98 P100 L3 is correct.
Step 2: Check other options for errors
M99 is for return, not call; order matters; M100 is not a command.
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