ST20-C1 datasheet(48/205 Pages) STMICROELECTRONICS

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4.9 Shifting and byte swapping

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4.8.1

Memory bit test and clear or set

Bits of a word in memory may be tested and set or cleared by the instruction

rmw. The

address of the memory word is held in Areg and a bit masks in Breg and Creg.

rmw

clears the bits of the memory word that are set in Creg, and then sets the bits of the

memory word that are set in Breg.The initial memory word is loaded into Areg, with

Areg pushed down to Breg and Breg pushed down to Creg.

4.9

Shifting and byte swapping

The shift and byte swapping operations are provided by the instructions listed in Table

4.11.

The shift operations (

shl, shr and ashr) shift the operand in Breg by the number of bits

specified b y the unsigned integer in Areg and put the result in Areg.

shl and shr fill the

vacated bit positions with zero bits, while

ashr fills the v acated bits with copies of bit

31, which is the original sign bit. If Areg is zero, the result is the initial value of Breg.

When the value in Areg is greater than the number of bits in the object being shifted,

the result of the operation is undefined. The data previously held in Creg is popped

into Breg, and the initial Breg is left in the Creg.

swap32 reverses the order of the bytes in Areg by swapping byte 0 with byte 3 and

swapping byte 1 with byte 2.

4.10 Function and procedure calls

The function and procedure call operations are provided by the instructions listed in

Table 4.12.

The primary instruction

fcall n calls a function or procedure. It stores the instruction

pointer (which holds the return address) in the word pointed to by the Wptr. The

operand to the call -

n - is added to the address of the next instruction to produce the

address of the first instr uction of the procedure or function being called. Since the call

address is relative, the code is relocatable.

Mnemonic

Name

shl

shift left

shr

shift right

ashr

arithmetic shift right

swap32

byte swap 32

Table 4.11 Shifting and byte swapping instructions

Mnemonic

Name

fcall

function call

jab

jump absolute

ajw

adjust work space

gajw

general adjust workspace

Table 4.12 Function and procedure instructions

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