Bios Rom - Panasonic JS-170FR Series Service Manual

2.1.4

BIOS ROM

A flash memory of 512K × 8 bits is used for the BIOS.
ROM's address decoding is undertaken by the chip set. The BIOS setting area is 000E0000h ~ 000FFFFFh or FFFE0000h ~
FFFFFFFFh that is the most significant area for the CPU. (Either address can be accessed.)
When used as a 512K type, continuous access becomes possible in the virtual memory area.
The flash memory is a write-enabled memory that requires command control by software when writing.
The BIOS ROM data bus is connected to the X bus since ROM's fan-out is small. It is then connected to the system data bus
(SD bus) via the buffer (HCT245).
Since this system uses the shadow memory method, the contents of ROM are written in the main memory when the BIOS is
started. After the system has been started, the CPU disconnects the ROM and works with the RAM.
The shadow memory method is effective to raise the processing speed by performing system's operation with the RAM the
access speed of which is higher than that of the ROM. It is possible to separate the programs that are not required in other cases
than system start. As a result, the work area can be expanded.
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