Unformatted text preview: etype origins of the code, such as 'carriage return', 'line feed', and 'bell'. The normal way to store an ASCII character in a computer is to put the 7-bit binary code into an 8-bit byte. Many systems extend the code using, for example, the 8-bit ISO character set where the other 128 binary codes within the byte represent special characters (for instance, characters with accents). The most flexible way to represent characters is the 16-bit 'Unicode' which incorporates many such 8-bit character sets within a single encoding. '1995' encoded as 8-bit printable characters is: 00110001 00111001 00111001 00110101 =31 39 39 3516 Printable characters ASCII ARM support for characters The support in the ARM architecture for handling characters is the unsigned byte load and store instructions; these have already been mentioned as being available to support small unsigned integers, but that role is rare compared with their frequency of use for transferring ASCII characters. There is nothing in the ARM architecture that reflects the particular codes defined by ASCII; any other encoding is equally well supported provided it uses no more than eight bits. However, it would be perverse these days to choose a different code for characters without having a very good reason. Data types 157 Byte ordering The above ASCII example highlights an area of some potential difficulty. It is written to be read from left to right, but if it is read as a 32-bit word, the least significant byte is at the right. A character output routine might print characters at successive increasing byte addresses, in which case, with 'little-endian' addressing, it will print '5991'. We clearly need to be careful about the order of bytes within words in memory. (ARMs can operate with either little- or big-endian addressing. See 'Memory organization'on page 106.) A high-level language defines the data types that it needs in its specification, usually without reference to any particular architecture that it may run on. Sometimes the number of bits used to represent a particular data type is architecture-depe...
View Full Document
This document was uploaded on 10/30/2011 for the course CSE 378 380 at SUNY Buffalo.
- Spring '09