> The memory chips respond by sending the contents of the selected
memory cell over the data bus to the CPU.
What does that ROM memory cell _physically look like_? How do we physically manipulate it to contain a 1 or a 0 (absence of something)?
What does that ROM memory cell _physically look like_? How do we physically manipulate it to contain a 1 or a 0 (absence of something)?
The memory cell itself might be a handful of transistors forming a bistable flipflop (for SRAM), or it might be a capacitor and transistor (for DRAM), or a floating gate and transistor (for EPROM), or just a wire and transistor (for mask-ROM).
Then there's PROM (Programmable Read-Only Memory), which is "empty" (either all 0, or all 1, I don't know the full details) but can be programmed once and that's it. Then there's EPROM (Erasable Programmable Read-Only Memory), which can be erased (by exposing the actual chip to UV light), then reprogrammed. Then there is EEPROM (Electrically Erasable Programmable Read-Only Memory) which can be electrically erased and reprogrammed. For each type of ROM, once programmed, it good for years, if not forever.
The programming of a PROM, EPROM or EEPROM usually consists of applying a higher than normal voltage to the chip on certain pins and is usually done in a separate device. How these chips works internally (how the gates are arranged, erased, programmed, etc.) is not something I know (being into software). This is just stuff I picked up over the years.
on a die, you wouldn't go through that trouble, you would just directly synthesize the zeros and ones.
but we don't really use ROMs that much anymore, they are usually persistent and programmable
Now here's the clever bit - there are diodes between the vertical and horizontal wires. When you pull a vertical wire to ground it pulls all the horizontal wires connected through diodes to ground, putting a 0 on those pins.
In a real "mask programmmed" ROM the grid is more square, so there are a lot of horizontal lines grouped in 8 bit bytes.
In an EPROM the diode is replaced by a little MOSFET. By applying a high voltage to its gate it'll stay charged basically forever, switching on and forming a "0" in the output.
I’m not an expert in modern hardware, but these are the basic principles I recollect. Happy to be corrected if this answer is dated :)
--The instruction set is the lowest level of user-visible software. The instruction set is implemented (in principle) by microcode.
--Microcode (if present) is a bunch of hardwired logic signals that cause data to move between subunits of the CPU in response to instructions. Register transfer specifications govern the design at this level.
--A CPU is a collection of subunits including registers, arithmetic units, and logic gates.
--Registers are collections of flip-flops.
--Arithmetic units are made of logic gates.
--Flip-flops are made from logic gates in feedback configurations to make them stateful. State machine theory governs the design of these circuits.
--Logic gates are stateless and made of transistors. Boolean algebra governs the design at this level.
--Transistors are made of NMOS or PMOS or bipolar silicon junctions. Device physics and electrical properties govern this level.
This abstraction hierarchy is useful when learning, but in the real world the abstraction layers are much blurrier: Everything's really just transistors, stateless stuff often has invisible state, electrical considerations matter at every level, etc.