ST20-C1 datasheet(10/205 Pages) STMICROELECTRONICS

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2.2 Instruction definitions

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represent addresses, signed or unsigned integers. Generally the arithmetic in signed.

In some cases it is clear from the context (e.g. from the operators used) whether a

particular object represents a signed or unsigned number. A subscripted label is

added (e.g. Areg

unsigned) to clarify where necessary.

2.2.5

Representing memory

The memory is represented by arrays of each data type. These are indexed by a value

representing a byte address. Access to the three data types is represented in the

instruction descriptions in the following way:

byte[

address]references a byte in memory at the given address

sixteen[

address]references a 16-bit half word in memory

word[

address]references a 32-bit word in memory

For all of these, the state of the machine referenced is that

before the instruction if the

function is used without a prime (e.g. word[

address]), and that after the instruction if

the function is used with a prime (e.g. word

′[address]).

For example, writing a value given by an expression,

expr, to the word in memory at

address

addr is represented by:

word

′[addr] ← expr

and reading a word from a memory location is achieved by:

′← word[addr]

Writing to memory in any of these ways will update the contents of memory, and these

updates will be consistently visible to the other representations of the memory. For

example, writing a byte at address 0 will modify the least significant byte of the word at

address 0.

Data alignment

Generally, word and half word data items have restrictions on their alignment in

memory. Byte values can be accessed at any byte address, i.e. they are byte aligned.

16-bit objects can only be accessed at even byte addresses, i.e. the least significant

bit of the address must be 0. 32-bit objects must be word aligned, i.e. the 2 least

significant bits of the address m ust be zero.

Address calculation

An address identifies a par ticular byte in memory. Addresses are frequently calculated

from a base address and an offset. For different instructions the offset may be given in

units of bytes or words depending on the data type being accessed. In order to

calculate the address of the data, a word offset must be converted to a byte offset

before being added to the base address. This is done by multiplying the offset by the

number of bytes per word, i.e. 4.

As there are many accesses to memory at word offsets, a shorthand notation is used

to represent the calculation of a word address. The notation

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