Page 348 - Embedded Microprocessor Systems Real World Design

Program Counter
The control store is driven by a progrum counter. This is just a binary counter that counts from
zero to however large the control store is. If the control store holds four instructions, the
program counter needs to be 2 bits wide. If the control store holds 1024 instructions, the
program counter needs to be 10 bits wide. The program counter is incremented each time
an instruction is executed in order to select the next instruction in the control store.

Opcodes
Say that the control store is 9 bits wide and we define the bits like this:

Bits 0, 1: select ALU function (00 = addition, 01 = negation, 10 = AND, 11 = OR).
Bits 2, 3: Select Zregister (00 = ZO, 01 = Z1, 10 = 22, 11 = 23)
Bits 4, 5: Select Y-register (00 =YO, 01 = Y1, 10 = Y2, 11 = Y3)
Bits 6, 7, 8: Select which register the result will be clocked into:
000 = zo, 001 = 21,010 = 22,011 = 23,
100 = YO, 101 = Yl, 110 = Y2,lll = Y3

The different combinations of bits that tell the machine what to do are called opcodes. Now,
say we want to write a program that will execute the following two operations:
Add Y1 to 22, putting the result in Y2
AND Y2 with 23, putting the result in Y3

The control store would contain the following words, based on the preceding bit
definitions:
Location Control Store
0 11001 1000
1 1 1110 1110

The program counter starts at zero (remember, zerebased addressing), and the output
of the control store causes the first operation to be executed. Then the program counter
increments to one and the second operation is executed.

Branching
Now we can write a program, up to the length of the control store, to do any operation
that our simple machine is capable of. But what happens when we get to the end of the
program? To handle this, we can expand our control store from 9 to 20 bits, as shown in
Figure D.5.
Of the added 11 bits, 10 wrap back around to the program counter as inputs, and one is
a branch control bit. When the branch control bit is 0, the program counter increments,
just as it did before. But when the branch control bit is 1, the program counter is loaded

Appendix D 329