Personal Computer Extended Memory Card With Direct Memory Access
Original Publication Date: 1987-Dec-01
Included in the Prior Art Database: 2005-Feb-02
The IBM PC AT operating system (DOS) requires that the Intel 80286 microprocessor run in real address mode. To access memory above 1 megabyte, the 80286 must run in protected virtual address mode. To switch from virtual mode to real mode requires a hardware reset of the 80286. This hardware reset from virtual to real mode requires approximately 500 microseconds. Various applications require transfers of data between memory addressed above 1 megabyte and memory below 1 megabyte. A DOS application must switch to virtual mode and back to real mode for this data transfer. The mode switching is performed with the BIOS interrupt 15 code. This hardware reset time will often be as long as the data transfer time.