The IBM mode character sets conform to a more standard version of ASCII. IBM divides into two ASCII systems, CG set 1 and CG set 2. These systems adopt the standard ASCII codes for values 0-127, with the extension of standard ASCII in 128-255. In IBM CG set 1, codes 128-159 are shadows of codes 0-31, and 160-255 are international char- acters, IBM graphic characters and scientific symbols. In CG set 2, codes 128-159 are assigned still more international characters, while codes 3-6 are assigned trump card symbols. For a summary of PETASCII and IBM ASCII codes, refer to Appen- dix A of this manual.
Secondary Addres.ses and ESCape Codes Although the original ASCII standard was designed to use the deci- mal numbers 1 through 127, computer and printer manufacturers soon extended this range (to 0 through 255) in order to make room for more features. On the MPS 1000 for example, the codes from 160 through 2 5 5 are used for graphic characters in Commodore uppercase I graphic mode, and graphic or international characters in IBM mode. Because even this extended range is not enough for all the features used on modern printers, the range is extended in many Commodore systems through the use of what are called secondary addresses.
The secondary address is used in Commodore BASIC to specify a printer function, such as setting the number of lines to be printed on a page or setting spaces between lines in the printer. This characteristic of Commodore BASIC will be discussed more in Chapter Four. In many other systems, including those compatible with the IBM mode of the MPS 1000 printer, the range is extended through special ESCape codes (the code is often printed with the first three letters capitalized-as in ESCape-or abbreviated as ESC) . With the ESCape code, for which decimal 27 is used, printers and computers are not restricted to only 256 instructions.
The ESCape code is a signal that the next code will be a printer control code instead of text to print. For example, if the printer receives the number 69, it prints a capital E because 69 is the ASCII code for that letter. If, how- ever, the printer receives a 27 just before the 69, it turns on emphasized mode, because ESCape "E" is the code for emphasized. You can see how important the ESCape code is by looking at Appen- dix B. There you will see that nearly every code the MPS 1000 uses in IBM mode is an ESCape code. 33