Definition of 24-bit

 

This addressing mode references up to 16 Mb of memory and thus requires 24 bit to specify. The IBM System 360 introduced in 1964 used this size address space. 24 bits (2 to the 24th power) represents 16777216 bytes and as is usually done, stated as 16 megabytes. The System 370 which came to the market in the early 1970's had 31 bit addressing with provisions to handle 24 bit programs from the prevoious 360 line without major effort. IBM was and still is well known for not leaving existing customers in the lurch when introducing upgraded processors. In the System 360 line of mainframe computers the LA (Load Address) instruction zeroed the high-order byte of the target 32 bit register and this caused complications when older software was being upgraded to use the full power of the System 370. A lot of the old 360 software was simply used as is and only new code took advantage of the 31 bit 370 addressing space. On the newer 31 bit processors a distinction (Above the Line & Below the Line) was made on whether a program used 24 or 31 bit addresses.


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