address space than is available in an internal memory address register. As integrated circuit memory became less costly, it was feasible to design systems with larger and larger physical memory spaces. Fewer than 16 address pins Microcontroller devices with integrated I/O and memory on-chip sometimes had no, or a small, address bus available for external devices. For example, a microcontroller family available with a 2 kilobyte address space might have a variant that brought out an 11 line address bus for an external ROM; this could be done by reassigning I/O pins as address bus pins. Some general-purpose processors with integrated ROM split a 16-bit address space between internal ROM and an external 15-bit memory bus. Some very early computers also had CPUs with fewer than 16 address pins: The MOS Technology 6507 (a reduced pin count version of the 6502) was used in the Atari 2600 and was limited to a 13-line address bus. 16 address bits, 16 address pins Most 8-bit general-purpose microprocessors have 16-bit address spaces and generate 16 address lines. Examples include the Intel 8080, Intel 8085, Zilog Z80, Motorola 6800, Microchip PIC18, and many others. These processors have 8-bit CPUs with 8-bit data and 16-bit addressing. The memory on these CPUs is addressable at the byte level. This leads to a memory addressable limit of 216 × 1 byte = 65,536 bytes or 64 kilobytes. 16 address bits, 20 address pins: 8086, 8088, 80186 & 80188 The Intel 8086 and derivatives, such as the 8088, 80186 and 80188 form the basis of the popular x86 platform and are the first level of the IA16 architecture. These were 16-bit CPUs with 20-bit addressing. The memory on these CPUs were addressable at the byte level. These processors could address 220 bytes (1 megabyte). 16 bit addresses, 24 address pins: 80286 The Intel 80286 CPU used a 24-bit addressing scheme. Each memory location was byte-addressable. This results in a total addressable space of 224 × 1 byte = 16,777,216 bytes or 16 megabytes. The 286 and later could also function in real mode, which imposed the addressing limits of the 8086 processor. The 286 had support for virtual memory. 32 bit addresses, 24 address pins The Intel 80386SX was an economical version of the 386DX. It had a 24-bit addressing scheme, in contrast to 32-bit in the 386DX. Like the 286, the 386SX can address only up to