Explanation and demonstration of the status lights on the front panel of the Altair 8800. Support material for this video is available at altairclone.com....
It might be worth re-shooting this video in 1080p -- the additional sharpness would really help for videos specifically about the front panel. Too bad TH-cam only lets big companies "replace" videos without having to delete and re-upload...
The Stack Register (SP) is a physical register inside the CPU that stores a pointer to a memory location where the stacked data is stored. The real data is stored in memory, but the locations where that data will be loaded is stored in the CPU. A interesting thing about Stack is that the data is stack backward. If you stack a byte in address 0xF100 , the next byte will be stacked in address 0xF0FF and the SP register decreases. Another interesting thing is that the z80 only stacks 2 bytes a time. You can't stack only one byte.
@@marcelusmeridius Where in memory is the stack data located, or is that something you have to setup, or is there a default location, if not specified?
On the original Altair, the front panel LEDs are driven directly from the corresponding bus signal. The WO bus line is an active low signal, so its LED is driven "backwards." Oddly, the memory protect status line is also active low and MITS did put an inverter in line to make that LED work as expected. As far as the WO LED is concerned, it could be an oversight, it could be they ran out of gates on the front panel board. They did fix the inverted WO LED on the 8800b.
A majority of microprocessor bus/control lines are Active Low as deramp5113 explained. Expect to see this a lot, the reason they are like this lies with the manufacturer but for the most part it is due to the TTL/NMOS/CMOS design of the chips that favor a negative output for the ports. Most RAM/ROM chips are designed around this especially with the Chip Enable lines. However the fact that the Altair displays this as an active low without inverting it is beyond me. The Intel 8080 is infamous for it's bad design choices, it's the only reason companies like Zilog and AMD exist.
Oh and if a panel light or control signal is indicated with a solid line above it's name that indicates an active low signal. Just throwing that out there.
do you have handy a "Front Panel Saver" program? like a screensaver.... but for the Altair... i have mine running "kill the bit" as a sort of demo mode to show it "doing something" but to tell you the truth, that bit going by reminds me a the old crystal police scanners from back in the early 80s.... maybe something that exercises all the lights..... (i have the 8800micro, not the clone....)
I have written a little program that looks really cool in my opinion. It only works when your memory doesnt get cleared in boot though. 000:012 012 012 012 024 302 000 000 010:012 012 012 012 034 302 000 000 020:301 036 370 303 000 000 You can change the speed by changing the Byte in octal Adress 22. The hoher it is the faster.
If the CPU has compleated a program it is supposed to stop executing. This is what the halt command do. It stops the processor until an interrupt occurs (in case interrupts are allowed). For example if you write a program that requieres some time and shouldn't be run in the step mode, it can end in a halt instruction, so the program runs and then just stops. Or if you add a keyboard it can wait until you pressed a key.
It's a true shame that your camera records at such a low quality. It is genuinely difficult to read the labels for the led lights on the front panel :'(
Before you continue with this you should memorize binary and hexadecimal not octal. There are several bitwise conversions/arithmetic that throw me off because you use octal.
Actually, I think and breathe in hex and binary. Octal is the difficult base for me. But... like the DEC PDP computers, this is an octal oriented machine. The front panel is laid out in octal, all original documentation uses octal, the assemblers provided by MITS all assume and use octal. So for historical accuracy - octal it is.
The octal base where used in Altair because the 8080 uses octal to form a instruction, for example: the two bits os some instructions says what the type of that instruction( load, move , store,jump) while the middle 3 bits and the last 3 bits refers to operands, usually registers. A,B,C, etc.
@@marcelusmeridius That's the case for some instructions (notably MOV), but not all. LXI for example is 00rp0001 where rp selects the register pair; that doesn't fit comfortably into octal notation but works great in hex. Intel's original documentation used hex almost exclusively. For more info see retrocomputing.stackexchange.com/q/12101/7208 .
@@deramp5113 Here's how you fix your front panel to work in hexadecimal: commons.wikimedia.org/wiki/File:Living_Computers_-_Altair_8800_(39802981903).jpg :-)
Very clear and thorough explanation. Thank you.
I would like this video to have a 1080p resolution
It might be worth re-shooting this video in 1080p -- the additional sharpness would really help for videos specifically about the front panel. Too bad TH-cam only lets big companies "replace" videos without having to delete and re-upload...
For your next video, can you do a description of the I.B.M. 360's front panel? ;)
Is the Stack Register a physical register inside the CPU or does the programmer setup up an area in memory to act as the Stack Register?
The Stack Register (SP) is a physical register inside the CPU that stores a pointer to a memory location where the stacked data is stored. The real data is stored in memory, but the locations where that data will be loaded is stored in the CPU. A interesting thing about Stack is that the data is stack backward. If you stack a byte in address 0xF100 , the next byte will be stacked in address 0xF0FF and the SP register decreases. Another interesting thing is that the z80 only stacks 2 bytes a time. You can't stack only one byte.
@@marcelusmeridius
Where in memory is the stack data located, or is that something you have to setup, or is there a default location, if not specified?
Jeff, SP (stack registers) starts pointing to 0 after a reset , but usually the programer need to setup stack as the fisrts steps of the program.
@@marcelusmeridius No, actually, the SP is not reset to 0000h by a RESET. It's random on power-up. I don't know if it's preserved across a RESET.
Is there a reason why the WO (Write Output) LED is inverted?
On the original Altair, the front panel LEDs are driven directly from the corresponding bus signal. The WO bus line is an active low signal, so its LED is driven "backwards." Oddly, the memory protect status line is also active low and MITS did put an inverter in line to make that LED work as expected. As far as the WO LED is concerned, it could be an oversight, it could be they ran out of gates on the front panel board. They did fix the inverted WO LED on the 8800b.
A majority of microprocessor bus/control lines are Active Low as deramp5113 explained. Expect to see this a lot, the reason they are like this lies with the manufacturer but for the most part it is due to the TTL/NMOS/CMOS design of the chips that favor a negative output for the ports. Most RAM/ROM chips are designed around this especially with the Chip Enable lines. However the fact that the Altair displays this as an active low without inverting it is beyond me. The Intel 8080 is infamous for it's bad design choices, it's the only reason companies like Zilog and AMD exist.
Oh and if a panel light or control signal is indicated with a solid line above it's name that indicates an active low signal. Just throwing that out there.
i'm 4 videos in and I still don't know what I'm doing with my life.
do you have handy a "Front Panel Saver" program? like a screensaver.... but for the Altair...
i have mine running "kill the bit" as a sort of demo mode to show it "doing something"
but to tell you the truth, that bit going by reminds me a the old crystal police scanners from back in the early 80s....
maybe something that exercises all the lights..... (i have the 8800micro, not the clone....)
I have written a little program that looks really cool in my opinion. It only works when your memory doesnt get cleared in boot though.
000:012 012 012 012 024 302 000 000
010:012 012 012 012 034 302 000 000
020:301 036 370 303 000 000
You can change the speed by changing the Byte in octal Adress 22. The hoher it is the faster.
what is the use of the halt command?
If the CPU has compleated a program it is supposed to stop executing. This is what the halt command do. It stops the processor until an interrupt occurs (in case interrupts are allowed). For example if you write a program that requieres some time and shouldn't be run in the step mode, it can end in a halt instruction, so the program runs and then just stops. Or if you add a keyboard it can wait until you pressed a key.
Why is the WO light inverted? Such a weird design decision
Actually quite a few of the 8080 pins are inverted logic.
It's a true shame that your camera records at such a low quality. It is genuinely difficult to read the labels for the led lights on the front panel :'(
you can read the lights at s2js.com/altair/sim.html.
Keep in mind this was uploaded in 2013 -- that's just how most cameras were a decade ago. TH-cam did not support HD at that point.
Before you continue with this you should memorize binary and hexadecimal not octal. There are several bitwise conversions/arithmetic that throw me off because you use octal.
Also Hex is nibble aligned to binary where octal is not and loading Instructions/Operands can be done in nibbles without conversion at all.
Actually, I think and breathe in hex and binary. Octal is the difficult base for me. But... like the DEC PDP computers, this is an octal oriented machine. The front panel is laid out in octal, all original documentation uses octal, the assemblers provided by MITS all assume and use octal. So for historical accuracy - octal it is.
The octal base where used in Altair because the 8080 uses octal to form a instruction, for example: the two bits os some instructions says what the type of that instruction( load, move , store,jump) while the middle 3 bits and the last 3 bits refers to operands, usually registers. A,B,C, etc.
@@marcelusmeridius That's the case for some instructions (notably MOV), but not all. LXI for example is 00rp0001 where rp selects the register pair; that doesn't fit comfortably into octal notation but works great in hex. Intel's original documentation used hex almost exclusively. For more info see retrocomputing.stackexchange.com/q/12101/7208 .
@@deramp5113 Here's how you fix your front panel to work in hexadecimal: commons.wikimedia.org/wiki/File:Living_Computers_-_Altair_8800_(39802981903).jpg
:-)