A microprogram memory, a read only memory, or volatile memory area of a processor is referred to, in which the micro program of a microprogram control station is stored in binary form as a microcode.
In the simplest case, the microcode of N micro instructions of the binary word length m is formed, i.e., the microprogram memory includes exactly n × m bits. The micro instruction memory is usually used only for reading the micro- commands, and may be embodied as read only memory ( ROM). For the purpose of subsequent optimization of the microcode, it is advantageous to design the control memory as a volatile memory. This memory area is the microprogram control unit to read microinstructions for controlling the calculator. The access speed is adapted to the processor processing power. Therefore, the microprogram memory are like the microprogram control and computational work on the same processor chip with the shortest possible connections. Access to the microprogram memory is done as with the micro-instruction bit length m is not necessarily a multiple of the size of one byte (8 bits). Consequently, the microprogram memory is intended primarily as a bit or microinstruction word - addressable memory.
Common memory sizes between 50 ... 500 kbit, the word width of a microinstruction between m = 10 ... 100, the microprogram address space between n = 3k ... 24k. The size of the microcode address space depends therefore on how many micro- instructions are needed on average to convert a machine instruction ( opcode ) into microinstructions; the word length according to how complex the simulatan running microprogram control is designed.
Typical memory sizes microcode- programmable mainframe from 1960 to 1980 to lead:
- IBM 360 (models 30, 40, 50, 65 ): 8k × 50 4K × 56, 2.75 × 88, 2.75 × 100 k
- IBM 370 (models 145, 155, 165, 125): 16k x 32, 8k × 72, 2.5 × 108 k, 20 k × 22
- Siemens 4004 ( Model 45, 151): 4k × 56 × 72 3k
- PDP-11 (models 05, 40, 45, 70): 0.25 k × 40 × 56 0.25 k 0.25 k × 65 × 65 0.25 k
The significance of the microprogram memory as microcode memory for controlling a virtual arithmetic logic unit will be explained on the educational way microcode simulator Micro. The estimated size of the microprogram memory for 1024 instructions, each with 49 bit is here 49 kbit (1k × 49 bit).