ST20C2 datasheet(18/212 Pages) STMICROELECTRONICS

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Most instructions that involve fetching data from or storing data into memory, use word

aligned addresses (i.e. bits 1 and 0 are set to 0) and load or store four contiguous

bytes. However, there are some instructions that can manipulate part of the bit pattern

in a word, and a few that use double words.

A data item that is represented in two contiguous bytes, is referred to as a 16-bit

object. This can be stored, either in the least significant 16-bits of a word location or in

the most significant 16 bits , hence addresses of such locations are 16-bit aligned (i.e.

bit 0 is set to 0).

A data item that is represented in two contiguous words is referred to as a 64-bit

object or a double word.

Similarly, a data item represented in a single byte is sometimes referred to as an 8-bit

object.

2.3

Signed integers and sign extension

A signed integer is stored in twos-complement format and may be represented by an

N-bit object. Most commonly a signed integer is represented by a single word (32-bit

object), but as explained, it may be stored, for example, in a 64-bit object, a 16-bit

object, or an 8-bit object. In each of these formats, all the bits within the object contain

useful information.

Consider the example shown in Figure 2.2, which shows how the value -10 is stored in

a 32-bit register, firstly as an 8-bit object and secondly as a 32-bit object. Obser ve that

bits 31 to 8 are meaningful for a 32-bit object but not for an 8-bit object. These bits are

set to 1 in the 32-bit object to preserve the negative sign of the integer being

represented.

Figure 2.2 Storing a signed integer in different length objects

11 11 0110

these bit values not related to integer value

bit position

11 11 0110

bit position

...

signed integer value (-10) stored as an 8-bit object (byte)

signed integer value (-10) stored as a 32-bit object (word)

ST20C2 数据表(PDF) 18 Page - STMicroelectronics