INT 15 - Amstrad PC1512 - WRITE DATA TO NON-VOLATILE RAM
        AH = 01h
        AL = NVRAM location (00h to 3Fh)
        BL = NVRAM data value
Return: AH = return code
            00h OK
            01h address bad
            02h write error
SeeAlso: AH=02h"Amstrad"

Format of NVRAM:
Offset  Size    Description
 00h    BYTE    time of day: seconds
 01h    BYTE    alarm time: seconds
 02h    BYTE    time of day: minutes
 03h    BYTE    alarm time: minutes
 04h    BYTE    time of day: hours
 05h    BYTE    alarm time: hours
 06h    BYTE    day of week, 1 = Sunday
 07h    BYTE    day of month
 08h    BYTE    month
 09h    BYTE    year mod 100
 0Ah    BYTE    RTC status register A
                bit 7: set if date/time being updated
                bits 6-4: time base speed, default 010 = 32768 Hz
                bits 3-0: interrupt rate selection, default 0110 = 1024 Hz
 0Bh    BYTE    RTC status register B (see below)
 0Ch    BYTE    RTC status register C (read-only)
                bit 7: IRQF flag
                bit 6: PF flag
                bit 5: AF flag
                bit 4: UF flag
 0Dh    BYTE    RTC status register D
                bit 7: battery good
 0Eh  6 BYTEs   time and date machine last used
 14h    BYTE    user RAM checksum
 15h    WORD    Enter key scancode/ASCII code
 17h    WORD    Forward delete key scancode/ASCII code
 19h    WORD    Joystick fire button 1 scancode/ASCII code
 1Bh    WORD    Joystick fire button 2 scancode/ASCII code
 1Dh    WORD    mouse button 1 scancode/ASCII code
 1Fh    WORD    mouse button 2 scancode/ASCII code
 21h    BYTE    mouse X scaling factor
 22h    BYTE    mouse Y scaling factor
 23h    BYTE    initial VDU mode and drive count
 24h    BYTE    initial VDU character attribute
 25h    BYTE    size of RAM disk in 2K blocks
 26h    BYTE    initial system UART setup byte
 27h    BYTE    initial external UART setup byte
 28h 24 BYTEs   available for user application
Note:   bytes 00h-0Dh are the same on the IBM AT as they are used/updated by
          the clock chip

Bitfields for RTC status register B:
 bit 7  clear if normal update, set if abort update
 bit 6  periodic interrupt enable
 bit 5  alarm interrupt enable
 bit 4  update end interrupt enable
 bit 3  square wave enable
 bit 2  date mode (clear = BCD, set = binary)
 bit 1  24-hour format
 bit 0  daylight saving time enable

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