ST20-C1 datasheet(32/205 Pages) STMICROELECTRONICS

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4.2 Loading and storing

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normally used for local variables on the stack while non-local addressing is normally

used for all other variables.

The primary instructions

ldl and stl perform loading and storing of local variables. For

example to load a value

x words above the Wptr and write to a location y words above

the Wptr:

ldl x;

stl y;

The primary instructions

ldnl and stnl perform loading and storing of non-local vari-

ables. For example, to load a value

x above a base address x_base and store to a

location

y words above y_base, where x_base and y_base are held in local variables:

ldl x_base; ldnl x;

ldl y_base; stnl y;

Note that for the purposes of this manual,

ld X denotes loading the value from a

variable

X, where X may be a local or non-local variable, so either ldl or ldnl may be

used as appropriate. Similarly

st X denotes storing a value into a variable X, where X

may be a local or non-local variable, so either

stl or stnl may be used.

4.2.3

Byte and half-word values

Instructions are provided for loading and storing byte and half-word variables. In each

case, the address is initially in the Areg and is incremented by the size of the object,

so that repeated loads and stores can be used to copy a block of memory.

The load instructions place the loaded value in the Areg, the incremented address in

the Creg and leave the Breg unaffected. The store instructions write the initial Breg

into memory at the address in the Areg, leaving the incremented address in the Breg,

the initial Creg in the Areg and the initial Breg pushed down to the Creg.

Byte loading and storing

lbinc loads the byte at the address in Areg, into the evaluation stack. lbinc replaces

the address in Areg with the byte stored at that address, treating it as an unsigned

integer by setting the twenty-four most significant bits in Areg to 0. The incremented

address is left in Creg.

sbinc writes the least significant b yte in Breg to the location addressed by Areg. The

address is incremented by 1 and put in the Breg.

Half-word loading and storing

lsinc and lsxinc load the half-word object at the address in Areg, into the evaluation

stack.

lsinc replaces the address in Areg with the half word, treating it as an unsigned

integer by setting the sixteen most significant bits in Areg to 0.

lsxinc is similar to lsinc,

but treats the half-word as a signed integer in twos-complement format, and hence

sign extends the representation by setting the sixteen most significant bits in Areg to

the same value as the most significant bit of the half-word object. Sign extension is

discussed in section 4.4.6.

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