bits

EE 308

Setting and Clearing Bits
using Assembly and C

To put a specific number into a memory location or register (e.g., to put 0x55 into PORTA):
- In assembly:
```
        ldaa    #$55
        staa    PORTA
```
- In C:
```
        PORTA = 0x55;
```
To set a particular bit of a register (e.g., set Bit 4 of PORTA) while leaving the other bits unchanged:
- In assembly, use the bset instruction with a mask which has 1's in the bits you want to set:
```
        bset    PORTA,#$10
```
- In C, do a bitwise OR of the register with a mask which has 1's in the bits you want to set:
```
        PORTA = PORTA | 0x10;
```
  or
```
        PORTA |= 0x10;
```
To clear a particular bit of a register (e.g., clear Bits 0 and 5 of PORTA) while leaving the other bits unchanged:
- In assembly, use the blcr instruction with a mask that has 1's in the bits you want to clear:
```
        bclr    PORTA,#$21
```
- In C, do a bitwise AND of the register with a mask that has 0's in the bits you want to clear:
```
        PORTA = PORTA & 0xDE;
```
  or
```
        PORTA = PORTA & ~0x21;
```
  or
```
        PORTA &= ~0x21;
```

Bill Rison
2001-02-12