Generally, when a CPU chip first receives power, it must be reset by
receiving a pulse on its RESET (or RST) pin. This is because when the power
supply is first powering up, even if it only takes a second or two, the CPU
has already received “dirty” power, because the power supply was building up
a steady stream of electricity. Digital logic chips like CPUs require precise
voltages, and they get confused if they receive something outside their
intended voltage range. Thus, as soon as the chip has powered up, it is reset
to bring it to a known starting condition. This is done automatically by
circuitry on the motherboard that performs a reset upon power-up. The RESET
pin (which is usually active-low) must be activated for a certain number of
clock cycles to reset the CPU. The reset circuit keeps the RESET signal
active for a moment, then disables it, at which point the CPU begins its act. (View Highlight)
It simply
executes instructions from memory. Ultimately, all the CPU really is, is a
chip which receives instructions, and then performs those instructions. (View Highlight)
The RAM is the CPU’s workspace, where
it temporarily stores data that it is currently working on. (View Highlight)
The ROM is the
permanent code that the CPU reads every time it is turned on; The ROM is
always the first code to get executed on the CPU. (View Highlight)
The CPU addresses memory
(both RAM and ROM) through the address bus, sending out a particular
combination of 1s and 0s on the address bus lines to choose a particular byte
of memory. The memory chips respond by sending the contents of the selected
memory cell over the data bus to the CPU. (View Highlight)
Every CPU has a particular point in memory where it begins reading
instructions after it has been reset. Some CPUs will simply jump to a set
point and begin executing the instructions there, while others actually use
what is called a “reset vector”, which means that it first checks a
particular point in memory for a number which is the memory address to begin
executing instructions at. (View Highlight)
As an example of this, the Z80 CPU immediately begins executing code from
memory address 0000 when it is reset. (View Highlight)
By
contrast, the 6502, another popular classic CPU, has a two-byte reset vector
located at memory addresses FFFC and FFFD (in hexadecimal). This means that
the ROM in a 6502-based computer must be at the top of the memory space.
The two bytes are stored backwards, and thus, if FFFC contains 00 and FFFD
contains B0, then the 6502 will jump to memory location B000 and start
executing instructions there (View Highlight)
There are two advantages to this system: First
of all, it gives the computer engineer some control over where the CPU begins
executing ROM code, and secondly, it leaves the bottom area of the memory
space (beginning at address 0000) free for RAM. (View Highlight)
The CPU contains a register called the instruction pointer (abbreviated IP)
which contains a number. The number in the IP is the memory address at which
the next instruction is to be performed. IP is incremented with each
instruction, and in the event of a JMP instruction (a jump instruction, which
tells the CPU to jump to another location and start running the instructions
there), IP is set to the jump location and then the CPU continues on its way
from there. (View Highlight)
The CPU’s instructions are sometimes called “opcodes”. They are
simply strings of binary 1s and 0s which together form an instruction (View Highlight)
on a standard Intel 80x86 CPU (such as a 486 or Pentium), the opcode
90h (or 10010000 binary) is a NOP (no operation) opcode. NOP is the simplest
instruction in any CPU, and it simply means to do nothing and go on to the
next instruction (View Highlight)
Regardless of where the CPU begins getting its instructions, the beginning
point should always be somewhere in a ROM chip. The computer needs startup
instructions to perform basic hardware checking and preparation, and these
are contained in a ROM chip on the motherboard called the BIOS. This is where
any computer begins executing its code when it is turned on. (View Highlight)
Once the BIOS code has been executed, what happens next depends entirely on
what is in the BIOS, although normally the BIOS will begin looking for a disk
drive of some kind and start executing the instructions there (which is
usually an operating system). (View Highlight)
How do the chips know when the CPU is addressing them, and when
it is not? A very popular solution to this is to use a converter chip,
usually a 3-to-8 converter, and occasionally a 2-to-4 converter is used. (View Highlight)
The 3-to-8 converter is simply a chip with three logic inputs and eight logic
outputs. Depending on which combination of inputs is on, a single of the
eight outputs will be activated (View Highlight)
The three inputs
are attached to the highest three lines of the address bus (A13 to A15 in a
CPU with a 16-bit address bus), and the eight outputs can now be used as chip
select signals. Very nearly every RAM or ROM chip in existence has a “Chip
Select” (CS) pin, which can enable or disable the chip, and whichever chip
receives the CS signal from the 3-to-8 converter will be the one that
responds to the memory access by the CPU. (View Highlight)
^rwID-464408228
New highlights added April 24, 2023 at 2:25 PM
because you are devoting 3 of your address bus lines to chip
selection, you have reduced addressing functionality within the actual chips.
However, this is not usually a problem. The remaining 13 address bus lines
give you 8K of memory space in each chip, which is usually enough for small
computers (View Highlight)
If you have 8 memory chips, each of them with 8K of memory in
them, then you have a full 64K of addressable memory, using the full capacity
of a CPU with a 16-bit address bus (View Highlight)
