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16 bit Date Extraction
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Information on FAT16

Date Code

Extracting the Date

The dates used in a File record are in a 16 bit Forward as shown below.

Most modern computers default to a year of 1980, so you need to add 1980 (0x07BCh) to the extracted year.

An example showing one way of extracting the Date from the 16 bit value.

; Extracts a File Date ; 00-04 - Day of the month (0-31) 5bits ; 05-08 - Month (1-12) 4bits ; 09-0F - Year less 1980 (ie, 0=1980) 7bits ; Pass AX = Date as File Date Word ; Returns AX=Year, BL=Day, BH=Month SubConvertWordToDate: PUSH ECX MOV CX, AX ; store the value while using AX SHL AL, 0x03 SHR AL, 0x03 MOV BL, AL ; BL = Day MOV AX, CX SHR AX, 0x03 SHR AL, 0x02 MOV BH, AL ; BH = Month MOV AX, CX SHR AH, 0x01 SHR AX, 0x08 ADD AX, 0x07BC ; AX = Year POP ECX RET

Creating a Date

There will be times when you need to save the date back to the disk.

; Creates a Date as a 16 bit value ; Pass AX=Year, BH=Month, BL=Day ; Returns AX=Date as 16 bits SubConvertDateToWord: PUSH ECX MOV CX, AX SHL CX, 0x01 ; leaves Year in CH ready to ADD MOV AH, BH ; Move Month to AH MOV AL, 0x00 SHR AX, 0x03 ADD AL, BL ; Add Day ADD AH, CL ; Add Year POP ECX RET