Bitwise Operations

Registers are compact. A single byte may contain several independent fields:

Bit 7 | Bit 6 | Bit 5 | Bit 4 | Bit 3 | Bit 2 | Bit 1 | Bit 0
------+-------+-------+-------+-------+-------+-------+-------
 UIP  |  DV2  |  DV1  |  DV0  |  RS3  |  RS2  |  RS1  |  RS0

The UIP bit is bit 7. To isolate it:

uint8_t uip = reg_a & 0x80;

To build the mask without hardcoding:

uint8_t mask = (uint8_t)(1u << 7);

Read Values

Use & to test whether a bit is present. The operation keeps only the bits that are set in both operands, so a mask with one bit set is enough to ask a yes-or-no question about that position. In C, the result does not need to be exactly 1 to be true; any non-zero value is true. This is why a helper such as bit_is_set should be clear about whether it returns the raw mask value or a normalized boolean-like result.

if (value & BIT(7)) {
  /* bit 7 is set */
}

Construct Write Values

Use | to set bits and & ~mask to clear bits. These two operations are the basic tools for building control words. They also make it possible to change one field while preserving neighboring fields, which is essential when a register contains several unrelated settings.

value |= BIT(2);
value &= (uint8_t)~BIT(2);

This lab mostly reads the RTC. Later labs use the same idea to construct timer control words, UART line-control values, and mouse/keyboard commands.

When you are debugging bitwise code, write the value in binary on paper or in a comment beside the test. Many mistakes are not arithmetic mistakes; they are position mistakes. Bit 0 is the least significant bit, so shifting by one moves to bit 1, not to the second byte or to a decimal digit.

TIP

Write masks from bit positions, then name the mask. Named masks document the device protocol and reduce off-by-one errors.