- The computer has the
capability to perform
a (limited) number operations
Sample instructions:
- Add integer values (i.e., 2's complement codes)
- Add floating point values (i.e., IEEE 754 codes)
- Subtract integer values (i.e., 2's complement codes)
- Subtract floating point values (i.e., IEEE 754 codes)
- and so on...
- Compare integer values (i.e., 2's complement codes)
- Compare
floating point values
(i.e., IEEE 754 codes)
- "Jump" to a given memory location
- The type of
operations that a
computer can
perform is
defined by:
- The structure of its Central Processing Unit (= CPU)
- All
commercially available
computers have a
similar CPU design
- We will study this commonly used CPU structure
- This is a detailed picture
of a computer system with
details of the
CPU and
memory
exposed:
I will explain the components in the CPU in more details now
The general structure of a CPU (found in all existing computers) is as follows:
- General Purpose Registers:
- very fast memory located inside the CPU
- is usually 10 to 50 times faster than main memory (located outside the CPU)
- used to store results of calculations
- ALU (Arithmetic and Logic Unit)
- performs arithmetic (+, -, *, /, %) and logic (AND, OR, NOT) operations
- takes input from general purpose registers and sometimes also from MBR (memory operand)
- produce output that will be stored in a general purpose register or sometimes in MBR (result is written to memory)
- certain outcome of the operation (negative, zero, overflow and carry) are stored in the condition flags in the PSR (see later)
- Communication unit:
- performs data transfer functions between CPU and main memory
- use two special purpose registers
in its task
- MAR
= Memory Address Register, contains the address of the memory
location that the CPU wants to transfer from/to.
- MBR = Memory Buffer Register, contains the data being transferred (MBR works like a temporally transfer depot)
- MAR
= Memory Address Register, contains the address of the memory
location that the CPU wants to transfer from/to.
- Control unit:
- controls the operation of all other components
- works like an orchestra conductor
- coordinates the registers, ALU and communication unit
so that they perform their function at the right moment to
achieve the correct functionality.
- Uses these special purpose registers
to perform its task:
- IR = instruction register
(contains the instruction code
of the current instruction
(that is being executed by the CPU)
- PC = program counter
(contains the address of the next
instruction that needs to be fetched)
- PSR =
processor status register
(contains the state information of
the processor - in cs255, we only care about the
N, Z, V, C flags in this register (later))
- IR = instruction register
(contains the instruction code
of the current instruction
(that is being executed by the CPU)
-
PSR in more detail
- The PSR consist of a number of
status indication bits (= flags)
to signal some conditions
- For assembler programmers, the
following 4 flags (bits) inside
the PSR will be important:
- N flag/bit =
1 means the result of the operation is
negative, 0 means >= 0
Example:
Consider the following subtraction in 2's complement code: 00000010 (= 2) 00000001 (= 1) - 00000001 (= 1) - 00000010 (= 2) ------------- -------------- 00000001 (= 1) 11111111 (= -1) The CPU will set the N flag as follows after the subtract operation: N = 0 N = 1
- Z flag/bit =
1 means the result of the operation is
equal to 0, 0 means not equal to 0
Consider the following subtraction in 2's complement code: 00000010 (= 2) 00000010 (= 2) - 00000001 (= 1) - 00000010 (= 2) ------------- -------------- 00000001 (= 1) 00000000 (= 0) The CPU will set the Z flag as follows after the subtract operation: Z = 0 Z = 1
- V flag/bit =
1 means the result of the operation
has caused an
overflow, 0 means no overflow
Consider the following addition in 2's complement code: 01111110 (= 126) 01111111 (= 127) + 00000001 (= 1) - 00000001 (= 1) ------------- -------------- 01111111 (= 127 (= 126 + 1)) 10000000 (= -128 (≠ 127 + 1 !!!)) The CPU will set the V flag as follows after the subtract operation: V = 0 V = 1 (there was an overflow - the number was too large)
- C flag/bit =
1 means the result of the operation has
generated a
carry/borrow, 0 means no carry/borrow
Consider the following addition in 2's complement code: 11111110 (= -2) 11111111 (= -1) + 00000001 (= 1) + 00000001 (= 1) ------------- -------------- 11111111 (= -1) 00000000 (= 0) 1 (carry) The CPU will set the Z flag as follows after the subtract operation: C = 0 C = 1
How does the computer use these N,Z,V,C flags:
- When we subtract
2 numbers
x − y:
- The N, Z, V, C flags will be set according to result of x − y
- From the values of the
N, Z, V, C flags, we can
tell whether:
- x == y (Z flag = 1 !!!)
- x ≠ y
- x < y
- x ≤ y
- x > y
- x ≥ y
(Exactly how to use the flag to do so is not important for the assembler programmer - like you.
You just need to know that the computer can tell the different outcome apart using these 4 flags)
- N flag/bit =
1 means the result of the operation is
negative, 0 means >= 0
- The PSR consist of a number of
status indication bits (= flags)
to signal some conditions
- Datapath:
- Datapath =
the circuitry
located inside the
CPU to
transfer the
data (= binary number or bits)
between
the CPU components
- Datapath =
the circuitry
located inside the
CPU to
transfer the
data (= binary number or bits)
between
the CPU components
- Example on how the
datapath is
used in
computation
(addition):
- The source data are
transferred to
the ALU:
- The CPU can now
perform the addition:
- The sum of the
addition can be
stored in a
register:
- The sum of the
addition can
also be
stored into
memory:
If you want to learn more about exactly how this works, you need to take CS355
- The source data are
transferred to
the ALU:
- System bus:
- System bus =
the connections between
the CPU and
the memory (= RAM)
- System bus is used to transfer data (= binary numbers or bits) between the CPU and the memory (RAM)
- System bus =
the connections between
the CPU and
the memory (= RAM)
- We have discussed the operation
of the system bus before
See: click here
- I can show you how the
CPU is
connected to the
system bus in
more details now
(that you have seen some details of the CPU):
Notice that:
- The MAR register of the
CPU is
connect to the
address bus
- The CPU will write the address into its MAR register to send the address onto the address bus
- The MBR register of the
CPU is
connect to the
data bus
- The CPU will
write the
data into its
MBR register to
send the
data (= binary number or bits)
onto the
data bus
(which will be written
to the memory)
- The CPU can also
copy the
data from
the data bus into its
MBR register
In this case, the CPU will read the data (= binary number or bits from memory !!!
- The CPU will
write the
data into its
MBR register to
send the
data (= binary number or bits)
onto the
data bus
(which will be written
to the memory)
If you want to learn more about exactly how this works, you need to take CS355
- The MAR register of the
CPU is
connect to the
address bus