ST20-C1 datasheet(35/205 Pages) STMICROELECTRONICS

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4 Using ST20-C1 instructions

constant from the

nth entry in the constant table stored at address constants_ptr the

following code would be used:

ldl constants_ptr ; ldnl n;

where the instruction

ldnl n is explained in section 4.2.2.

This code sequence only takes 2 bytes, provided

constants_ptr is less than 16 words

from the work space pointer address and there are no more than 16 word-length

constants. At worse it is unlikely to take more than 4 bytes. Hence, if a constant takes

4 or more bytes to load using

ldc then this sequence often improves code compact-

ness — especially if the constant is used more than once.

4.3.4

Assignment

Single words, half words and bytes may be assigned using the load and store instruc-

tions described in section 4.2.

Word assignment

x and y are both single word variables and e is a word valued expression then word

assignments are compiled as

x = y

compiles to

ld y; st x;

x = e

compiles to

e; st x;

Byte assignment

a and b are both single byte variables and e is a byte valued expression then byte

assignments are compiled as

b = a

compiles to

address(a); lbinc; address(b); sbinc;

b = e

compiles to

e; address(b); sbinc;

where

address(variable) is the address of variable. Forming addresses is discussed in

section 4.5.

Half word assignment

a and b are both half-word variables and e is a half-word valued expression then

half-word assignments are compiled as

b = a

compiles to

address(a); lsinc; address(b); ssinc;

b = e

compiles to

e; address(b); ssinc;

where

address(variable) is the address of variable. Forming addresses is discussed in

section 4.5.

4.4

Arithmetic

This section describes the use of the arithmetic instructions except for the multiply-

accumulate instructions, which are described in Chapter 5, and forming addresses,

which is described in section 4.5. Boolean expression evaluation is discussed in

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