16 megabytes of memory. The Motorola 68000 had a 24-bit address space, allowing it to access up to 16 megabytes of memory. 32 bit addresses, 32 address pins The 386DX had 32-bit addressing, allowing it to address up to 4 gigabytes (4096 megabytes) of memory. The Motorola 68020, released in 1984, had a 32-bit address space, giving it a maximum addressable memory limit of 4 GB. All following chips in the Motorola 68000 series inherited this limit. 32 bit addresses, 36 address pins: Pentium Pro (aka P6) The Pentium Pro and all Pentium 4s have 36-bit addressing, which resulted in total addressable space of 64 gigabytes, but it requires that the operating system support Physical Address Extension. 64 bit computing Modern 64-bit processors such as designs from ARM, Intel or AMD are typically limited to supporting fewer than 64 bits for RAM addresses. They commonly implement from 40 to 52 physical address bits (supporting from 1 TB to 4 PB of RAM). Like previous architectures described here, some of these are designed to support higher limits of RAM addressing as technology improves. In both Intel64 and AMD64, the 52-bit physical address limit is defined in the architecture specifications (4 PB). Operating system RAM limits CP/M and 8080 addressing limit The first major operating system for microcomputers was CP/M. This operating system was compatible with Altair 8800-like microcomputers, made by Gary Kildall in conjunction with the programming language PL/M, and was licensed to computer manufacturers by Kildall's company Digital Research after it was rejected by Intel. The Intel 8080 used by these computers was an 8-bit processor, with 16-bit address space, which allowed it access up to 64 KB of memory; .COM executables used with CP/M have a maximum size of 64 KB due to this, as do those used by DOS operating systems for 16-bit microprocessors. IBM PC and 8088 addressing limit In the original IBM PC, the basic RAM limit is 640 KB. This is to allow for hardware addressing space in the upper 384 KB (upper memory area (UMA)) of the total addressable memory space of 1024 KB (1 MB). Ways to overcome the 640k barrier, as it came to be known, involved using special addressing modes available in the 286 and later x86 processors. The 1 MB total address space was a result of the 20-bit address space limit imposed on the 8088 CPU. Using the color