ST20-C1 datasheet(42/205 Pages) STMICROELECTRONICS

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4.5 Forming addresses

42/205

Access to a component of an array can be split into two sections; first the address of

the component must be constructed, and then the transfer of data to or from that

component must be performed.

Evaluating a subscript

Array subscripts can be evaluated efficiently using the

smul or mul instruction. If array

A has been declared by

int

A[S1] .. . [Sn];

where

Si (i = 1..n) are the dimensions, then one way of arranging this in memory is to

have all elements of the array in a contiguous block. For the purposes of this section,

suppose that the elements in the last dimension are stored adjacently; otherwise

change the order of the dimension subscripts. For example Figure 4.1 shows the

elements of a particular three dimensional array (Array) stored in this way.

Figure 4.1 A possible method of storing an array of integers

If an access is required to the following array element

A[e1] ... [en]

then the code to evaluate the subscript is

e1;

ldc S2; mul; e2; add;

ldc S3; mul; e3; add;

.. .

ldc Sn; mul; en; add;

For example to evaluate the subscript for element Array[x][y][z], (where Array

is declared as in Figure 4.1) the code sequence is

ld x;

ldc 2; mul; ld y; add;

ldc 3; mul; ld z; add;

If x is 1, y is 0 and z is 2, then this evaluates to 8, which as can be seen from Figure

4.1, is the correct offset from the base of the array.

Array[1][1][1]

Array[1][1][0]

Array[1][1][2]

Array[1][0][1]

Array[1][0][0]

Array[1][0][2]

Array[0][1][1]

Array[0][1][0]

Array[0][1][2]

Array[0][0][1]

Array[0][0][0]

Array[0][0][2]

int Array[

2][2][3];

Increasing

memory

addresses

Contiguous

locations for

words in

memory

space

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