Microcode

1. Microcode Source: Digital Computer Electronics (Malvino and Brown)

2. Micro-code • Micro-code is the instructions at the lowest level, closest to the hardware. • Any higher-level instructions (including assembly/machine language) must be converted to a lower level. • A single machine-language instruction (like Load Accumulator A) typically consists of several micro-code instructions.

3. Where is microcode stored? • It used to literally be wired in (hence the term “hard wired”). • Typically it stored in ROM. • If the code is stored in EEPROM, it can be changed; this is known as microprogramming. • But this is something one does on a rare occasion. • Sometimes referred to as “firmware,” an intermediate between software and hardware.

4. Machine language • A level above micro-code. • The instructions are numbers, which really are the addresses of the micro-code instruction in ROM. • Mnemonic version of machine language is called assembly language.

5. Assembly is a mnemonic

6. Getting down to hardware’s level • High level programs are translated into assembly language or machine language by a compiler. Assembly language programs are translated into machine language by an assembler. • Each processor has its own unique machine language. Thus code must be rewritten or at least recompiled to run on different processor (different hardware).

7. A simple design • Next we will show a computer design that uses the basic “bus architecture” • A bus is a basically just wires through which data travels from one part of a computer to another. • Usually it’s implied the path is shared by a number of parts. • (One also talks about buses in networks.)

8. Bus Keyboard encoder Input port 1 Accumulator Flags ALU Input port 2 Prog. counter TMP Mem.Add.Reg. B Memory C MDR Output port 3 Display Instr. Reg. Output port 4 Control

9. Keyboard encoder: converts key pressed into corresponding string of bits (ASCII) Input port 1: where keyboard data is entered, usually contains some memory (a buffer) where data is held until the processor is ready for it Input port 2: where non-keyboard data is entered Input ports

10. The program counter points to the current line of the program (which is stored in memory) This design shows arrows connecting the “PC” to and from the bus, why? If the next instruction to be executed is not the next line of code in memory, such as If Loops Subroutines, functions, etc. Program counter

11. MAR (Memory Address Register) holds the address of the memory location being read from or written to Not necessarily same as program counter Memory (RAM): the place where data and instructions are stored MDR (Memory Data Register) holds the data that is being read from or written to memory Bi-directional connection to bus for reading and writing MAR, MDR and Memory

12. Instruction register holds the instruction that is currently being executed. A given line of assembly or machine language code involves several micro-code instructions, the instruction register holds onto the instruction until all of the micro-instruction steps are completed Instruction Register

13. Executes the program at the lowest level. Sends signals to the control pins of all the devices involved. These lowest level instructions are in ROM. Each assembly-level instruction has a numerical counterpart (machine language); the numerical counterpart is the address of the microcode for that instruction stored in ROM. Not shown, controller connects to everything. Controller/Sequencer

14. Accumulator: register used in conjunction with the ALU. Data upon which arithmetic or logic operations will eventually be performed is stored here; also the results of these are stored here. ALU (Arithmetic Logic Unit) where operations that change the data (as opposed to just moving it around) are done. Accumulator and ALU

15. Flags are output from the ALU that are distinct from data (data output goes to Acc. A) For example, A carry from an addition An indication of overflow These are needed for program control or to indicate possible errors The result of a logical comparison (<, >, =) These are needed for control (ifs, loops, etc) Flags

16. TMP is the other register used in conjunction with the ALU; the distinction is that answers are generally sent to Accumulator A. B and C are additional registers used for holding data temporarily. They allow additional flexibility and reduce the amount that must be written to memory. TMP, B and C

17. Output port a connection to the “outside world” Usually includes a buffer This design has to one for displayed output and a second for other output (e.g. storage) Output ports

18. Let us now examine the steps involved in the assembly (machine language) instruction Load Accumulator A Micro-code

19. What do you mean by Load • There are different types of Loads • Load • Instruction and address • Associated data is the address of data that is to be put in Acc. A • Load immediate • Instruction and data • Associated data is actual data to be sent directly to Acc. A • Load indirect • Instruction and address of address • Associated data is an address. Found at that address is another address. Then at that second address is the data to be placed in Acc. A.

20. Fetch Cycle • Address State: the value of the program counter (which recall is the address of line of the program to be performed) is put into memory address register. • Increment State: the program counter is incremented, getting it ready for the next time. • Memory State: the current line of the program is put into instruction register (so Control knows what to do).

21. Execution cycle (Load Acc. A) • The remaining steps depend on the specific instruction and are collectively known as the execution cycle. • Recall the instruction consisted of a load command and an address. A copy of the address is now taken over to the memory address register. • The value at that address is loaded into Accumulator A. • For the load command, there is no activity during the sixth step. It is known as a "no operation" step (a "no op" or "nop").

22. Bus Keyboard encoder Input port 1 Accumulator Flags ALU Input port 2 Prog. counter TMP Mem.Add.Reg. B Memory C MDR Output port 3 Display Instr. Reg. Output port 4 Control

23. Data Movement • Many of the micro-code steps involve moving data and addresses to various locations (registers, memory locations, etc.). • The information is often, but not always, sent over the bus. • So information must be put on and taken from the bus.

24. Load and Enable • With memory, one talks about reading and writing. • With registers and the bus, one talks about enabling and loading. • Enabling: placing a value from a register on the bus. • Loading: placing a value from the bus into a register.

25. Register Control pins • A register that takes values off of the bus (e.g. the Memory Address Register, MAR) will need a “load” control pin • It does not always take the value on the bus, instead it takes the current value on the bus when the load pin is “activated” • “Active high”means the action is performed when the pin is high • “Active low” means the action is performed when the pin is low

26. The clock pin • The clock is another control pin (sometimes called a timing pin) which determines when a register takes the value on the bus • The load input determines if the register takes the value • The clock input determines when the register takes the value

27. The clock • A binary clock: 10101010101010101010 • Each cycle (01) should take the same amount of time (the time for a cycle: the period) • The number of cycles in a second is called the frequency • “on the edge:” many registers load on the clock’s edge • Positive edge: as 0 goes to 1 • Negative edge: as 1 goes to 0

28. Enable • The reverse of a load is an enable, this is when a device places a value on the bus • A register that places values on the bus (e.g. the buffer associated with an input port) must have an “enable” control pin • Again enabling may be active high or active low

29. “Only one bus driver” • Only one item can place its value on the bus (“drive the bus”) at a time. WHY? • Suppose two items drive the bus and that they have different values • Then there would be a direct connection (the bus is essentially just wire) between a high voltage and a low voltage • Since wire offers little resistance, there would be a very large current – a.k.a. a short • Large currents destroy digital circuits

30. Three-State logic • If a device is not driving the bus, it must be effectively disconnected from the bus • Otherwise the short problem • Thus we need three-state logic • High • Low • Disconnected

31. Tri-State buffer • “Disconnecting” all devices except the “bus driver” from the bus is done with tri-state buffers • These are not shown in our diagram and are distinct from chips • Thus we won’t find the kind of “enable” control pins discussed here on chips

32. Other control pins • Items involved in data manipulation (as opposed to simply data movement) will require additional control pins • For example, the program counter needs to be incremented • Thus additional control pins are required • These pins are sometimes also referred to as “enable” pins, as they enable a particular action

33. ALU control • The primary data manipulator is the ALU • The 74181 is a simple ALU • It has an M select to choose between logic and arithmetic functions (M) • It has a set of S selects (S0, S1, S2, S3) to choose among the various functions of that type

34. 74181 ALU Chip Data in Data out Data in Control pins

35. Micro-code is • Micro-code is 1’s and 0’s stored in ROM • The ROM output is connected to control pins • For example, one micro-code instruction is to take the value from the program counter to the memory address register • So send active signals to “enable the PC” and “load the MAR”