ROM memory is ideal for storing the BIOS program in the PC, which is essentially a very basic boot program. This boot program is very compact, and can reside a very small amount of memory (less than 2MB). ROM memories include Programmable ROM (PROM), Erasable Programmable ROM (EPROM), Electrically Erasable Programmable ROM (EEPROM) and others. EEPROMs are now used for the BIOS because they can be erased and re-written electrically, which allows BIOS update programs to burn-in a new boot program into the chip. This is what is called 'flashing the BIOS'.

From their inception, microcomputers were designed to be inexpensive, low-end systems that required the use of low-cost components, and system memory has always been implemented using the less expensive (and slower) DRAM. When the PC was introduced, DRAM speeds were more than adequate to handle the 4.77MHz bus of the 8086/8088, and even the faster 80286 processor (up to 12MHz, or 80ns). With the introduction of the 80386 processor, clock speeds of 20MHz, 25MHz and even 33MHz became possible, which existing DRAMs could not keep up with.

To hide the speed mismatch between processor and main memory, designers began to put a small amount of SRAM memory on the motherboard, running at the system bus speed, to hold the most recently used data. Even though SRAM is inherently much faster than DRAM, it didn't take long for the processor to outrun the motherboard cache as well. When the 80486 was introduced, it included 8K of SRAM cache embedded in the chip, which ran at processor speed and was called Level 1 (L1) cache, while the motherboard cache was called L2. This 'memory hierarchy' continues to be used with today's high-performance systems.