Specifications

PC/II+p Board Technical Reference Manual 61

7.21.4 Real-Time Clock – Interrupt 1Ah Functions

The BIOS supports AT compatible real-time clock functions using software interrupt 1Ah. In addition, the

PC/II+p BIOS supports the following functions using the software interrupt 1Ah, which provide read and write

access to bank 0 and bank 1 SRAM memory in the real-time clock, and read access to the unique serial

number encoded in the real-time clock chip.

1. FUNCTION 0FFh – WRITE AND READ BANK-0 SRAM

This function writes a byte to RTC SRAM Bank 0, or reads a byte from RTC SRAM Bank 0.

MOV AH,0FFh

MOV DL,<RTC bank 0 register number>

MOV DH,<Direction> Bit 0 = 0 (WRITE), Bit 0 = 1 (READ)

Bit 1-7 = Unused

MOV AL,<8-Bit Value to be Written> Used if DH.0 = 0, Unused if DH.0 = 1

INT 1Ah

-- returns here with

/FX.CF = 1 (Error)

/FX.CF = 0 (No Error)

/AH = Destroyed

/AL = value read from RTC or written to RTC>

2. FUNCTION 0FBh – WRITE AND READ BANK-1 EXTENDED SRAM

This function writes a byte to RTC Bank 1 Extended SRAM, or reads a byte from RTC Bank 1 Extended

SRAM.

MOV AH,0FBh

MOV DL,<RTC bank 1 register number> 0 – 7Fh

MOV DH,<Direction> Bit 0 = 0 (WRITE), Bit 0 = 1 (READ)

Bit 1-7 = Unused

MOV AL,<8-Bit Value to be Written> Used if DH.0 = 0, Unused if DH.0 = 1

INT 1Ah

-- returns here with

/FX.CF = 1 (Error)

/FX.CF = 0 (No Error)

/AH = Destroyed

/AL = value read from RTC or written to RTC>

3. FUNCTION 0FCh – READ RTC SERIAL NUMBER

This functions reads the silicon serial number that is embedded in the RTC chip. Each RTC chip is

manufactured to contain a unique serial number.

MOV AH,0FCh

LES DI,<pointer to seven-byte buffer that will receive the serial number field>

INT 1Ah

-- returns here with

/FX.CF = 1 (Error)

/FX.CF = 0 (No Error)

/[ES:DI+0] = Silicon Serial Number Byte 1 through Byte 6

/[ES;DI+6] = Silicon Serial Number CRC byte