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ST9 USER GUIDE

3.3.1.5 Bit Operations

Microcontrollers are often used for controlling inputs and outputs on a single-bit basis, in order

to read the state of a contact, switch a relay on or off, etc. Because of this and because the

data is stored in bytes, instructions for bitwise manipulation of data are welcome.

The ST9 provides instructions to load, and, or, exor, set, clear, complement and test single

bits. These are bld, band, bor, bxor, bset, bres, bcpl, btset.

To designate a single bit in a byte, the notation .n is used. For example, r0.3 means bit 3 of r0.

Here are examples of bit manipulation instructions:

bld

r0.2, r6.4

; bit 4 of r6 copied to bit 2 of r0

bld

r0.3, !r6.0

; complement of bit 0 of r6 copied to bit 3 of r0

band

r0.2, r0.3

; r0.2 contains now (r6.4) and not (r6.0)

bor

r0.2, r2.7

bset

r0.0

; bit 0 or r0 set to 1

bcpl r0.1; bit 1 of r0 is complemented

All the above instructions act on single working registers. If the source operand is preceded by

‘!’, the complement of the source bit is used.

To test a bit, to condition a later jump, we have already described the tm and tcm instructions.

There is another instruction, btset, that can act on either a single or a double working register,

and that sets the Z bit of the FLAGR register if the designated bit is zero. After which, the bit

is set to one. You can use this instruction in an interrupt service routine to test for a request

and acknowledge it in a single instruction.

Warning. Don’t use the bit manipulation instructions directly on bidirectional ports. To avoid

unwanted modifications to the port output register contents, use a copy of the port register,

then transfer the result with a load instruction to the I/O port. (Refer to the Input/Output Bit

Configuration section in the Device Datasheet for more details.)

3.3.1.6 Test and Jump

The btjf and btjt instructions test if a bit is set or cleared respectively and branch to another

program location if true. For example:

btjt

r1.5,Lampon

bset

r1.5

; switch lamp on

Lampon:

...

; continuation of the program

Two instructions are well-suited for implementing lookup tables. These are cpjfi and cpjti. They

compare a byte in a register with a byte pointed to by a register pair, and increment the pointer

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