December 1991/Using 1KHz Interrupts On AT Clones/Table 1

Table 1 CPU Register Definitions for SET INTERVAL Service

AX     0x8300
CX     Interval count in microseconds (high word)
DX     Interval count in microseconds (low word)
ES:BX  Wait flag address: a pointer to a byte in the calling program's
       memory. Bit 7 of this byte will be set when the interval
       expires.