1 COMP541 Sequencing and Control -- II Montek Singh April 5, 2007

2Topics  Still on Chapter 8  Last class Hardwired control Hardwired control  Now Microprogrammed control of multiplier Microprogrammed control of multiplier

3 Microprogrammed Approach  Control values stored in a memory  Job of instructions is to generate control signals to datapath and output

4 Nomenclature and Characteristics  Word of memory called microinstruction  The set of instructions called microprogram  Sometimes in ROM, sometimes loadable  Often wide word

5 Recall Datapath

6 Microprogrammed Control Unit  Control Address Register (CAR) equivalent to PC  Sequencer Part of instruction sent to next-address generator to determine next instruction addr Part of instruction sent to next-address generator to determine next instruction addr

7 Control Data Register  Pipelining approach to break up the delay in the addr gen and ROM  Not used in example

8 Status Bits  Notice that they go only to sequencer  Can only affect next control word  So, conditional output boxes not allowed in this architecture

9 ASM – old vs. new

10Microinstruction  Word format  Addresses of potential next instructions  Fields for next instruction selection  Fields for datapath control

11 Datapath Control Signals  Doesn’t include load reg inst.  Look at ASM to see where they are asserted

12 Mapping to Microinstruction  Have only 4 signals  Could encode (2 bits?)  Would cost a decoder Just a design tradeoff Just a design tradeoff This design has tiny ROM anyway This design has tiny ROM anyway

13 Sequencer Design  Probably most important part of this process  This design provides 2 addrs SEL field and control logic choose one SEL field and control logic choose one  Other possibility is one addr field Choice is to go to next sequential addr (like PC), or Choice is to go to next sequential addr (like PC), or Using control signals go to addr specified Using control signals go to addr specified

14 SEL Field

15Result  5 words in ROM  ROM is 12 bits wide  Design next

16 Microprog Design for Mult Mux chooses addr1 or addr2 Mux control from datapath status and external signals. Next slide

17 Detail of Control

18Microprogram

19 Is it necessary?  Microprogramming less useful now that HLLs used for design  Can use Verilog to generate very complex state machine

20Today  Looked at ways to implement microprogrammed control for the multiplier

21Next  Look at computer Sequencing Sequencing Pipelining Pipelining  We’ll also return to look at programmable logic We’d skipped earlier We’d skipped earlier