Questions tagged [microcode]
The microcode tag has no summary.
10 questions
16
votes
4
answers
6k
views
What was the last x86 processor that didn't have a microcode layer?
In the earlier days of microprocessors instructions were hard-wired, i.e. a particular instruction triggered circuitry that was mostly (if not completely) implemented for that instruction. I believe ...
- 19.2k
8
votes
2
answers
581
views
What is the relation between external clock and internal states in the 68000?
(I'm assuming a memory cycle of 500 ns, without wait states.)
According to the 68000 bus diagram, there are 4 CPU cycles for a memory cycle, so an external frequency of 8 MHz. However, things are ...
- 1,598
8
votes
3
answers
7k
views
What is the MOS 6502 doing on each cycle of an instruction?
So, first off, I am kind of a noob with emulators and the 6502.
Summarise the problem
If we for example take the instruction ADC Immediate ($69) which adds the accumulator to an immediate value and ...
user20750
10
votes
3
answers
584
views
Were the VAX eXtended Function Control (XFC) instructions ever used commercially?
The DEC VAX has a set of 256 instructions set aside for user addition via microcode edits. Were extension sets ever commercially produced and sold, or was this functionality mostly used in-house?
...
- 275
9
votes
4
answers
2k
views
Why did later CPUs use microcode instead of PLA's?
If you look at a die photo of a 6502, about forty percent of the chip is taken up by what's obviously microcode, both by its regular structure and by the obvious need for such from the instruction set,...
- 65.3k
1
vote
3
answers
2k
views
How do multi-byte instructions work?
Let's say you have a hypothetical 8-bit processing machine with 256 bytes of memory. You want to load the value stored in memory location $f2 into a register.
However, with a data bus of just 8 bits, ...
- 341
20
votes
5
answers
3k
views
Why are PDP-7-style microprogrammed instructions out of vogue?
DEC, and at least some of their computers, especially those in the 18-bit family and 12-bit family, had these opr instructions, which contained many bitfields which encoded something like "...
- 41.2k
16
votes
2
answers
2k
views
6502 branch offset calculation
This question expands on How does the 6502 implement its branch instructions?
I'm working on a cycle accurate VHDL implementation on an FPGA. I have much of the program logic already written, but I ...
- 163
11
votes
4
answers
10k
views
Was the IBM 5100 ever used for codebreaking?
There's a rumor that there's something embedded in the IBM 5100 which makes it useful for codebreaking.
What is the legitimacy of that claim?
- 219
31
votes
5
answers
9k
views
How was microcode implemented in retro processors?
How was microcode implemented in retro processors such as the Z80 or 8080?
Was the microcode standard (for example a manual for the processor outlining all possible micro-instructions and the ...
- 2,045