ST20C2 datasheet(10/212 Pages) STMICROELECTRONICS

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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[]), and that

after the instruction if the

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

′[]).

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:

Areg

′← 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, e.g.

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

Reading and writing in this way cannot be used to access on-chip peripherals.

Reading or writing to memory addresses between

PeripheralStart and PeripheralEnd

will have undefined effects.

Data alignment

Each of these 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 and 64-bit objects must be word aligned, i.e. the 2 least significant

bits of the address must 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, words or double words depending on the data type being accessed. In

order to calculate the address of the data, the 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 in the particular units being used. So, for example, a word offset is

converted to a byte offset by multiplying it by the number of bytes in a word (4 in the

case of the ST20).

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

represent an address which is offset by

x words (4x bytes) from register. For example,

in the specification of

load non-local there is:

Areg

′← word[Areg @ n]

Here, Areg is loaded with the contents of the word that is n words from the address

pointed to by Areg (i.e. Areg

+ 4n).

In all cases, if the given base address has the correct alignment then any offset used

will also give a correctly aligned address.

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