RT/CMOS RAM I/O Operations
During I/O operations to the RT/CMOS RAM addresses, you should
mask interrupts to prevent other interrupt routines from changing the
RT/CMOS address register before data is read or written. After I/O
operations, you should leave the RT/CMOS address and NMI mask
register (hex 0070) pointing to status register D (hex 00D).
Attention
The operation following a write to hex 0070 should access hex 0071;
otherwise, intermittent failures of the RT/CMOS RAM can occur.
Writing to the RT/CMOS RAM requires the following:
1. Write the RT/CMOS RAM address to the RT/CMOS address and
NMI mask register (hex 0070).
2. Write the data to the RT/CMOS data register (hex 0071).
3. Write the address, hex 0F, to the RT/CMOS and NMI mask
register; this leaves hex 0070 pointing to the shutdown status
byte (hex 0F).
4. Read address hex 0071 to restore the RT/CMOS.
Reading from the RT/CMOS RAM requires the following steps:
1. Write the RT/CMOS RAM address to the RT/CMOS and NMI
mask register (hex 0070).
2. Read the data from the RT/CMOS data register (hex 0071).
3. Write the address, hex 0F, to the RT/CMOS and NMI mask
register; this leaves hex 0070 pointing to the shutdown status
byte (hex 0F).
4. Read address hex 0071 to restore the RT/CMOS.
2-20 System Board
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