Questions tagged [8080]

For questions about the Intel 8080 8-bit microprocessor

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14
votes
7answers
3k views

Intel 8080-based home computers

I'd like to build an FPGA replica of a home computer based on the Intel 8080. I am not looking to design my own computer, since I would like to tap into an existing software base. Were there any ...
23
votes
2answers
2k views

Details of video memory access arbitration in Space Invaders

I am working on an FPGA implementation of the original Space Invaders arcade machine and I'd like to implement access arbitration between the CPU and the video system. I can imagine several ways of ...
4
votes
3answers
257 views

Can someone explain this algorithm used to compute the auxiliary carry flag?

I'm debugging my 8080 emulator and I've gone the route of comparing it to the output of another tested emulator to see where I went wrong. When calculating the auxiliary carry flag, I used this ...
37
votes
1answer
6k views

Fixing obscure 8080 emulator bug?

I'm using the cpudiag.bin from http://www.emulator101.com/files/cpudiag.bin to test my emulator. I've already implemented the CP/M CALL function at 0x05, offset the program at 0x100 and fixed the ...
14
votes
4answers
3k views

Why do the i8080 I/O instructions take a byte-sized operand to determine the port?

I thought the i8080 had 8 16-bit IN ports and 8 16-bit OUT ports. Doesn't that mean the instructions IN d8 ; only index 0-7? OUT d8 can only take 8 possible values? What would it be indexing ...
1
vote
1answer
153 views

Where can I find and download emulator of Altair 8800 with Intel 8080 processor for Windows?

I would like to look back to the very beginning of personal computer when Paul Allen and Bill Gates started their debut. So I need to find an emulator of the MIT Altair 8800 with Intel 8080 processor ...
3
votes
2answers
324 views

Test emulated 8080 CPU without an OS?

I've found a few 8080 CPU test suites available, but they all assume CP/M or a similar OS is already present and running - they call BDOS for I/O, etc. I'm working towards being able to run CP/M on my ...
5
votes
3answers
409 views

Comparing Signed Numbers on Z80 (8080) in Assembly

How do you compare two signed numbers in Z80 assembly? According to this site, To compare stuff, simply do a CP, and if the zero flag is set [then] the argument[s] were equal, else if the carry ...
3
votes
3answers
288 views

Did the original Intel peripherals use latches as input registers?

This is more a question to satisfy my curiosity. When looking at the original timing diagrams for reading and writing (e.g. for the 8255 and the 8250), these signals are never expressed in function ...
2
votes
1answer
457 views

What is the difference between an 8080 and an 8051? [closed]

What are the features of the Intel 8051 architecture that allow it to successfully act as a microcontroller? Hypothetically can the general-purpose 8-bit architecture like Intel 8080 do everything the ...
7
votes
5answers
402 views

What stops an IO device from driving the databus longer than the CPU expects?

For example, consider the case of handling an INT interrupt signal in the Z80. When the CPU makes the IORQ signal active, it expects the requesting IO device to place a vector onto on the databus. And ...
2
votes
1answer
399 views

Source code of front-panel entered Altair 8800 programs

I'm looking for the source code of short programs for the MITS Altair 8800 (let's say < 50 instructions to make the question more specific), so they can be entered by using only the front panel. ...
3
votes
3answers
1k views

How can a Z80/8080/6502 use VGA?

I've come across a couple of projects that manage to get VGA working with the Z80 and similar CPUs: Veronica (6502 CPU, VGA signal generation by AVR) ZC160 (Z80 CPU, VGA signal generation by GAL) c-...
1
vote
4answers
435 views

Intel 8080 and Altair 8800. 256 I/0 ports, but only 7 free RST (interrupt subroutine) - how it works?

I have looked this video - Altair 8800 - Interrupt Acknowledge Cycle and have few questions (I have read wikipedia Intel 8080 article, Altair 8800 Operator's manual, Charles Petzold "Code" and few ...
27
votes
4answers
10k views

How did 8 bit processors perform 16 bit arithmetic?

How did 8 bit processors such as the Z80 and 8080 perform 16 bit arithmetic? They have an 8 bit data bus, so how does the ALU perform 16 bit arithmetic on the register pairs? Z80 architecture diagram ...
17
votes
2answers
2k views

The start of x86: Intel 8080 vs Intel 8086?

Why is it said that all modern Intel processors of the x86 family are said to descent from the Intel 8086 and not the Intel 8080? From the Wikipedia article on the Intel 8086, The 8086 gave rise to ...
9
votes
3answers
900 views

Intel 8080 - Behaviour of the carry bit when comparing a value with 0

I'm in the process of writing an emulator for the Intel 8080. The description of the CMP instruction from Intel's 8080 programming manual (see page 20) says the following: The specified byte is ...
10
votes
1answer
328 views

Why was the 8080's JumP immediate instruction placed where it was?

On the 8080 there are many instructions with parallel forms. Three of these are CALL, RET and JP, which all have conditional and immediate forms. Note: I am using Z80 terminology because that is what ...
1
vote
1answer
422 views

Why did this Hitachi 6301 assembler routine only work once?

I am trying to learn assembler on the various retro machines I own, or am about to buy. Today I tried converting this 8080 routine (which worked, repeatedly): | START: | MVI A,0 | 8000 | 3e | | ...
3
votes
1answer
595 views

Carry flag in 8080/8085 subtraction

How is the carry flag set during a subtraction by the 8080 or 8085 processor? For addition, the carry flag is set if the result is greater than 255 (i.e. if it overflows). From the programmer's ...
24
votes
4answers
4k views

What's the difference between “opcode” and “instruction” in this Zilog ad?

In a Zilog ad from 1976 comparing the Z80 and Intel 8080, the following table is presented: What is the difference here between "Instructions" and "OP Codes"? In my experience, the two terms are ...
4
votes
1answer
765 views

How was the Microsoft PDP-10 8080 emulator developed? [closed]

Altair BASIC, the first version of Microsoft BASIC, was developed using an Intel 8008 emulator, modified to emulate an Intel 8080, running on a PDP-10 computer. This emulator was originally designed ...
23
votes
1answer
2k views

What is the relative code density of 8-bit microprocessors?

When RAM is at a premium, as it was in the old days, a greater code density of an instruction set can be a substantial advantage. (Click saver: Code density refers loosely to how many microprocessor ...
17
votes
3answers
3k views

DIY Project using 1970s 8-bit CPU, is it possible?

I've recently really wanted to try and design a computer system around a retro 8-bit CPU such as the Zilog Z80 or the Intel 8080, and I would appreciate some sanity checks! I was inspired by Ben Eater'...
5
votes
0answers
159 views

Manual for DIAGNOSTICS II V1.2 - CPU TEST SUPERSOFT ASSOCIATES

I am building a 8080 emulator. I found some useful test suites here. The emulator passes the first test suite (8080/8085 CPU Diagnostic, version 1.0, by Microcosm Associates), but not the second ( ...
12
votes
1answer
268 views

What is the difference between i8080 and i8080a?

There is some info over the net regarding 'some i8080 bugs were fixed in i8080a', but the exact bugs and differences are not usually disclosed. So, what is the exact difference between i8080 and ...
14
votes
10answers
3k views

Were people building CPUs out of TTL logic prior to the 4004, 8080 and the 6800?

I've just finished reading Charles Petzold's book, Code: The Hidden Language of Computer Hardware and Software. In it Charles explains building relays into gates, gates into logic components, and ...
21
votes
4answers
3k views

Why did Pokémon Red have so many overflow bugs?

(This question also applies to other Game Boy games, but Pokémon seems to be one of the best-documented.) Pokémon Red & Blue (Red & Green in Japan) is a pair of Game Boy games about capturing ...
38
votes
2answers
7k views

How did the Z80 instruction set differ from the 8080?

The Zilog Z80 microprocessor, known for its use in the ZX Spectrum, was designed to be a backwards-compatible extension to the Intel 8080 processor. It introduced several new instructions to the 8080'...
6
votes
1answer
395 views

What exactly were those 808X computers that would switch to 80386 upon Ctrl-Alt-Del?

A while ago I've been told that some generic-brand PC, (yet packed on an IBM5150 case) had the ability to become a 80386 upon pressing Ctrl-Alt-Del (warm reboot) ? I guess there might have been (for ...
5
votes
3answers
1k views

LCD Displays with the Intel 8080

A colleague of mine mentioned that in college he wrote a slot machine program on an Intel 8080 using an LCD display. What kind of display communication protocol existed on the Intel 8080? How was ...
11
votes
1answer
523 views

8080/8085 Instruction Test Suite

(This question was inspired by Comprehensive test suites for MOS 6520 PIA / 6522 VIA / 6526 CIA) Is there a comprehensive test suite for the Intel 808X series of processors? Something that would ...
9
votes
1answer
742 views

What are the technical differences between the Midway 8080 and Taito 8080 hardware boards?

In my research on 1970s-era arcade machines, I've noticed that both Midway and Taito created games which used Intel's 8080 microprocessor. However, each company had their own host boards which ...
15
votes
2answers
2k views

What was the first arcade game to use a microprocessor instead of discrete logic?

Many authors credit Space Invaders as the first blockbuster game to have been written with an onboard microprocessor (Taito used Intel's 8080 for game logic), but there is also references to Taito's ...
8
votes
2answers
730 views

Intel 8080 Read/Set Interrupt Mask Instructions

According to Intel's 8080-8085 Assembly Language Programming guide, the OP Code 20 corresponds to the Read Interrupt Mask (RIM) instruction, and 30 corresponds to the Set Interrupt Mask (SIM) ...