In conjunction with the spare command codes, the I/O interrupts, and the I/O interrupt code registers, the I/O instructions provide a framework within which the user can implement his system interfaces. The particulars of the system interfaces outside of this framework (such as dedicated memory locations, channel register definitions, command code assignments/definitions, multiple channel priorities, page register access, etc.) are not included in this standard.
The input instructions transfer data from an external I/O device or an internal special register to a CPU general register. This command is used to read data from peripheral devices, timers, status word, fault register, discretes, interrupt mask, etc. A full description of the input instructions is given in the instruction repertoire.
The output instructions transfer data from a CPU general register to an external I/O device or special register. This command is used to write data to peripheral devices, discretes, start and stop timers, enable and disable interrupts and DMA, set and clear interrupt requests, masks and pending interrupt bits, etc. A full description of the output instructions is given in the instruction repertoire.
Input/output commands are classified as mandatory, optional, reserved, or spare. Mandatory I/O commands must be implemented as defined. Optional I/O commands must be implemented as defined, if implemented. Reserved I/O commands must not be implemented. Spare I/O commands may be implemented as required by the application. Attempted execution of an unimplemented optional or spare I/O command or a reserved I/O command shall cause the illegal I/O command fault to be set in the fault register (FT5) causing a machine error interrupt.
Input/output command words shall be fully decoded. "TBDs" in input/output instruction descriptions refer to parameters to be determined by the application system requirements. Within these classifications, the use of the command is defined in the instruction description.
The I/O command space shall be divided into 128 channels. Up to 512 commands within each channel group (256 input and 256 output) may be used with each I/O interface. Table IX lists the 128 I/O channel groups. The attempted execution of an unimplemented I/O command shall cause bit 5 of the fault register to be set, generate a machine error interrupt, and abort to completion.
Input/output level 1 and level 2 interrupts are available to the user. Either interrupt level or both may be implemented for an interface as defined by the particular application specification. The interrupts shall be used in conjunction with the input/output interrupt code registers to provide I/O channel to process communications. Two levels of interrupts allow easy differentiation of normal reporting from error reporting.