IOXT

IOXT instruction is used for programmed IO. IOXT is a privileged instruction in normal cases but if there is no OS running and the paging system is turned off then IOXT is an unprivileged instruction. IOX and IOXT are the only instructions used to communicate with external instructions.

IOXT transfers data between a device I/O register and the CPU's A register.

The T register is used as IO address. If bit 0 of the T register is zero then IOXT reads the value from the IO register into register A. If bit 0 is one then IOXT writes the contents of register A to the IO register.

Addressing of a non existing device results in a IOX error.

See also Programmed IO and IOX instruction
Ref. Programmed I/O, chapter 6.3 ND-100 Functional Description