The Registers Section of SimHYMN
The registers section represents the main registers in
the Central Processing Unit for the SimHYMN simulator as well as the two
flags used in some jump instructions.
The PC is the
Program Counter and contains 5 bits as well as a display of the value of
those bits. The 5 bits are shown here as 5 zeros in blue with yellow
background to the right of PC.
As with all registers (or memory words) you can change the PC by either clicking on bits
(which will change them to the opposite value) or by entering a value in
the text box to the right of the bits. In the case of the PC, the values you enter in
the text box right of the bits must be 1 or 2 digit hexadecimal numbers.
(At the moment, 00 indicates the value of the PC in hex.) The PC is used by the simulated PC
to hold the memory address of the next instruction to be executed.
The IR is the
Instruction Register. When an instruction is fetched from memory it is
placed into the IR
and, then, executed. You would never wish to edit the IR value. It will always be
changed as soon as the simulator starts.
The AC is the
Accumulator. All of the "action" takes place there. When a data value is
"loaded" from memory, it is placed into the AC. When a value is stored to
memory, it is the value in the ACwhich
is stored. Addition and Subtraction are done with the accumulator and a
value from memory and the result is placed in the AC - replacing the old value
that was there. When the AC
value changes, it resets the Zero
Flag and the Positive
Flag to proper values. For example, if the value loaded
into the AC were
00011011, a positive number, the Positive
Flag would become true and the Zero Flag would become false.
If the value loaded into the AC
were 11101110, a negative 2's complement number, the Zero Flag and Positive Flag would both be set
false. (As you can see, the Zero
Flag is currently true, indicating that the AC contains 0. Similarly, the Positive Flag is currently
false since 0 is not positive.)
Thus, the Zero Flag
and the Positive Flag are
intimately connected with the value in the AC.