1. A Discussion on Assemblers
Mnemonic instructions, such as LOAD 104, are easy for humans to write and understand. Also labels can be used to identify particular memory locations.
They are impossible for computers to understand.
Assemblers translate instructions that are comprehensible to humans into the machine language that is comprehensible to computers
We note the distinction between an assembler and a compiler: In assembly language, there is a one-to-one correspondence between a mnemonic instruction and its machine code. With compilers, this is not usually the case.
Assemblers create an object program file from mnemonic source code (assembly program) in two passes.
During the first pass, the assembler assembles as much of the program as it can, while it builds a symbol table that contains memory references for all symbols in the program.
During the second pass, the instructions are completed using the values from the symbol table.
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Lecture

2. A Discussion on Assemblers
Mnemonic instructions or alphanumeric name
Label or Memory location name
Consider our example program at the right.
Note that we have included two directives HEX and DEC that specify the radix of the constants.
The first pass, creates a symbol table and the partially-assembled instructions as shown (ie. doesn’t know X is located at address 104).
Also after the first pass, the translated instructions are incomplete
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Lecture

3. A Discussion on Assemblers
After the second pass, the assembler uses the symbol table to fill in the addresses and create the corresponding machine language instructions
After the second pass, it knows X is located at address 104 and that is totally translated to machine code
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Lecture

4. Extending Our Instruction Set
So far, all of the MARIE instructions that we have discussed use a direct addressing mode.
This means that the address of the operand is explicitly stated in the instruction.
It is often useful to employ a indirect addressing, where the address of the address of the operand is given in the instruction
If you have ever used pointers in a program, you are already familiar with indirect addressing.
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Lecture

5. Extending Our Instruction Set
We have included three indirect addressing mode instructions in the MARIE instruction set.
The first two are LOADI X and STOREI X, where X specifies the address of the operand to be loaded or stored.
In RTL :
It would be the same conceptually for AddI, SubI, JumpI and JnS
MAR  X
MBR  M[MAR]
MAR  MBR
AC  MBR
MAR  X
MBR  M[MAR]
MAR  MBR
MBR  AC
M[MAR]  MBR
LOADI X
STOREI X
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Lecture

6. Extending Our Instruction Set
Our first new instruction is the CLEAR instruction.
All it does is set the contents of the accumulator to all zeroes.
This is the RTL for CLEAR:
AC  0
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Lecture

7. A Discussion on Decoding
As mentioned earlier, the control unit causes the CPU to execute a sequence of steps correctly
There are control signals asserted on various components in making the components active
A computer’s control unit keeps things synchronized, making sure that bits flow to the correct components as the components are needed.
There are two general ways in which a control unit can be implemented: hardwired control and microprogrammed control.
With microprogrammed control, a small program is placed into read-only memory in the microcontroller.
Hardwired controllers implement this program using digital logic components. There is a direct connection between the control lines and the machine instructions.
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Lecture

8. A Discussion on Decoding
Your text provides a complete list of the register transfer language (or RTN) for each of MARIE’s instructions.
The RTL or RTN actually defines the microoperations of the control unit.
Each microoperation consists of a distinctive signal pattern that is interpreted by the control unit and results in the execution of an instruction.
The signals are fed to combinational circuits within the control unit that carry out the logical operations for the instruction
Recall, the RTL for the Add instruction is:
MAR  X
MBR  M[MAR]
AC  AC + MBR
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Lecture

9. A Discussion on Decoding
Each of MARIE’s registers and main memory have a unique address along the datapath (0 through 7).
The addresses take the form of signals issued by the control unit.
Let us define two sets of three signals.
One set, P2, P1, P0, controls reading from memory or a register,
and the other set consisting of P5, P4, P3, controls writing to memory or a register.
Let’s examine MARIE’s MBR (with address 3)
Keep in mind from Ch 2 how registers are configure using flip-flops
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Lecture

10. A Discussion on Decoding - MBR
The MBR register is enabled for reading when P0 and P1 are high
The MBR register is enabled for writing when P3 and P4 are high
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Lecture

11. A Discussion on Decoding
We note that the signal pattern just described is the same whether our machine used hardwired or microprogrammed control.
In hardwired control, the bit pattern of machine instruction in the IR is decoded by combinational logic.
The decoder output works with the control signals of the current system state to produce a new set of control signals.
Unique output signal corresponding to the opcode in the IR
Produce the series of signals that result in the execution of the microoperations
Produces the timing signal for each tick of the clock (sequential logic used here because the series of timing signals is repeated) – for tick, a different group of logic can be activated
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Lecture

12. A Discussion on Decoding - ADD
Timing signal added with instruction bits produce required behavior
Bit pattern for the Add = 0011 instruction in the IR.
Result Here
Control lines and bits controlling the register functions and the ALU
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Lecture

13. A Discussion on Decoding
The hardwired approach is FAST, however, the control logic are tied together via circuits and complex to modify
In microprogrammed control, the control can be easier modified
In microprogrammed control, instruction microcode produces control signal changes.
Machine instructions are the input for a microprogram that converts the 1s and 0s of an instruction into control signals.
The microprogram is stored in firmware, which is also called the control store.
A microcode instruction is retrieved during each clock cycle.
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Lecture

14. A Discussion on Decoding
All machine instructions are input into the microprogram. The microprogram’s job is to convert the machine instructions into control signals.
The hardwired approach, timing signals from the clock are ANDed using combinational logic circuits to invoke signals
In the microprogram approach, the instruction microcode produces changes in the data-path signals
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Lecture