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There are three principal aspects of assembly language programming: the machine instructions, the assembler, and the operating system.
The machine instructions are the primitive operations with which we write programs. Learning the instruction set means learning what operations are performed by each instruction. Programming is the art or science of combining these operations to accomplish some particular task.
The machine instructions, like everything else in a computer, are in binary. The assembler is a program that translates the mnemonic names by which we refer to instructons into the binary form that the computer recognizes. The assembler also does a variety of other chores that are essentially bookkeeping. There are several assemblers for the PDP-10, which differ in various ways; what they all have in common is the PDP-10 machine instructions, which are described in this file. Everything else about how to use a particular assembler is documented under that assembler. The assembler used most on ITS is called MIDAS; *Note MIDAS: (MIDAS).
The operating system is a special program that handles all input and output and which schedules among user programs. For its own protection and the protection of other users the operating system places various restrictions on user programs. User mode programs are resticted to memory assigned to them by the operating system; they may not perform any machine input-output instructions, nor can they perform several other restricted operations (e.g., HALT instruction). To facilitate user input-output and core allocation the operating system provides various system calls (UUO or JSYS operations) by which a user program can communicate its wishes to the system. Essentially all programs except the operating system itself are run as user mode programs. Editors, assemblers, compilers, utilities, and programs that you write yourself are all user mode programs.