How CMOS RAM stores year in one byte

Question

In this website, I find that the year in CMOS RAM is stored in just one byte.

It is known that one byte only holds 0 to 255, so I write this code to get what 09H store:

assume cs:code,ss:stack

stack segment
    db 64 dup (0)
stack ends

code segment
    start:
    mov ax,stack
    mov ss,ax
    mov sp,64

    mov al,09H
    out 70H,al
    in al,71H

    mov ah,4CH
    int 21H
code ends

end start

I get a return value 17H from port 71H, so does it mean CMOS RAM stores year by 1994 + offset value?

If so, what does number 1994 mean? In the future, how CMOS RAM stores the year more than 2249(1994+255)?

Answer 1

The register 09h of the CMOS stores the current year in the current century.
Its value ranges from 0 to 99.

----------R09--------------------------------  
CMOS 09h - RTC - YEAR  
Desc:   (BCD 00-99, Hex 00-63)  
Notes:  BCD/Hex selection depends on Bit 2 of register B (0Bh) 

It is encoded either as BCD (values 0x00 to 0x99) or as plain binary (values 0x00 to 0x63).
The encoding is determined by the bit2 of register 0xb

----------R0B--------------------------------  
CMOS 0Bh - RTC - STATUS REGISTER B (read/write)  

Bitfields for Real-Time Clock status register B:  
Bit(s)  Description     (Table C002)  
 7      enable cycle update  
 6      enable periodic interrupt  
 5      enable alarm interrupt  
 4      enable update-ended interrupt  
 3      enable square wave output  
 2      Data Mode - 0: BCD, 1: Binary  
 1      24/12 hour selection - 1 enables 24 hour mode  
 0      Daylight Savings Enable - 1 enables  

---

In my system register 0xb has value 0x2 (24-h enabled, BCD date).
Register 0x9 reads 0x17 for 2017, which is correct, in my frame of reference, for my timeline.