I ran into a question:
in digital system with micro-programmed control circuit, total of
32 distinct pattern operation signal is
450. if the
micro-programmed memory contains
1K micro instruction, by using
Nano memory, how many bits is reduced?
1) 22 Kbits 2) 23 Kbits 3) 450 Kbits 4) 450*32 Kbits
I read in my notes, that
(1) is true, but i couldn't understand how we get this?
Micro instructions are stored in the micro memory (control memory). There is a chance that a group of micro instructions may occur several times in a micro program. As a result the more memory space isneeded.By making use of the nano memory we can have significant saving in the memory when a group of micro operations occur several times in a micro program. Please see for nano technique ref:
Micro-memory The micro memory only concerns how we organize whats in the control memory. However when we have big instruction sets, we can do better than simply storing all the instructions. We can subdivide the control memory into "control memory" and "nano memory" (since nano is smaller than micro right ;) ) This is good as we don't waste a lot of valuable space (chip area) on microcode.
The concept of nano memory is derived from a combination of vertical and horizontal instructions, but also provides trade-offs between them.
The motorola M68k microcomputer is one the earlier and popular µComputers with this nano memory control design. Here it was shown that a significant saving of memory could be achieved when a group of micro instructions occur often in a microprogram.
Here it was shown that by structuring the memory properly, that a few bits could be used to address the instructions, without a significant cost to speed. The reduction was so that only the upper log_2(n) bits are required to specify the nano-address, when compared to the micro-address.