BDA

BDA : BIOS DATA AREA

BDA 存放System Bios 會使用到的重要的資料，

範圍 : 40:00 ~ "50" : 00 (Segment)

重要的Register :

40:1A Keyboard, Head of buffer pointer  (word)

40:1C Keyboard, End of buffer pointer (word)

40:1E Keyboard buffer (16 words) 

Keyboard Buffer can only holds 15 words.

When the buffer is full, additional keys are discarded by the INT9.

An error beep is generated for each discard.

40:6C Timer Ticks Count (Dword)

40:0E Extended BIOS Data Area Segment

EBDA 範圍 : EBDA:0 ~ EBDA:11D

EBDA is to hold data for a motherboard mouse port, hard disk parameters, and disk track buffers.

CMOS

The system contains a small amout of CMOS memory that retains its data while the system power is off.

CMOS chip also has a Real-Time Clock that keeps current time.

Accessing 限制

必須關閉interrupts (include NMI(port 70h bit 7) ) when access (read/write) CMOS

因為要確保在寫入index(70h port)時，別的中斷不會access CMOS

另外是read 時，也需要check Bit 7 of Register A，

Bit 7 of Register A 代表RTC updating ，

//loop for RTC updating

do {
          Status = BDA_CMOS->Io.Write ( 
                              BDA_CMOS,
                              EfiCpuIoWidthUint8,
                              0x70,
                              1,
                              &REG_A_OFFSET);
          Status = BDA_CMOS->Io.Read ( 
                              BDA_CMOS,
                              EfiCpuIoWidthUint8,
                              0x71,
                              1,
                              &DATA);
        } while (((DATA & 0x80) == 0x80)); 

RTC

A real time clock alarm is a feature that can be used to allow a computer to 'wake up' after shut down to execute tasks every day or on a certain day.