Explore the CMOS and RTC: Read System Clock with BIOS INT 0x1A

CMOS, RTC, and INT 0x1A

The CMOS RAM chip (indexed through ports 0x70/0x71) stores configuration bytes and a battery-backed clock. BIOS exposes RTC reads through INT 0x1A. For example, function AH=0x02 returns the current time: CH hours, CL minutes, DH seconds in BCD on many firmware builds.

Reading the clock via BIOS

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• INT 0x1A, AH=0x00: Read system clock count (rolls at midnight)
• INT 0x1A, AH=0x02: Read time of day
• INT 0x1A, AH=0x04: Read date (CX year, DH month, DL day)
• Port 0x70: CMOS address register (bit 7 disables NMI during access)
• Port 0x71: CMOS data register

CMOS also holds drive types, memory size fields (legacy), and boot order on older setups. Values are packed BCD or binary depending on the register.

BCD conversion

RTC bytes are often binary-coded decimal: the high nibble is tens, the low nibble is ones. Add 0x30 to each nibble or use a small conversion routine before printing ASCII digits. Reading register B flags tells you if daylight saving or 24-hour mode is active. NMI masking via port 0x70 bit 7 prevents spurious interrupts during multi-byte CMOS reads.

Why for this exercise

You will call INT 0x1A to read date and time and format them for display. This exercise requires understanding CMOS indexing and the BIOS register conventions for RTC services in real mode.