M/Command A

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command A or a

 Currently, the letter sequence A is the prefix for several commands: 

ASSIGN or assign or a or A

Currently, the letter sequence A is the abbreviation for the Command ASSIGN. This Command is part of the next M Standard.
This command is not implemented by all vendors.

ABLOCK or ablock or ab or AB

 Currently, the letter sequence AB is the abbreviation for the Command ABLOCK. This Command is part of the next M Standard.
This command is not implemented by all vendors. 

ASTART or astart or asta or ASTA

 Currently, the letter sequence ASTA is the abbreviation for the Command ASTART. This Command is part of the next M Standard.
This command is not implemented by all vendors.

ASTOP or astop or asto or ASTO

 Currently, the letter sequence ASTO is the abbreviation for the Command ASTOP. This Command is part of the next M Standard.
This command is not implemented by all vendors.

AUNBLOCK or aunblock or aunb or AUNB

 Currently, the letter sequence AUNB is the abbreviation for the Command AUNBLOCK. This Command is part of the next M Standard.
This command is not implemented by all vendors.
=== Former Non-Standard Command ===
The letter A used to be the abbreviation for the Command ASK.  No Standard M implementation is required to implement this command. Non-Standard M systems may include the Command ASK.