I'm writing an Intel 8080 emulator and I have a few questions about the 8080 chipset, but I want to make sure they are on-topic before posting on the main site. They are mostly arcade machines which some form of an 808X chip.
How many types of 8080 boards were produced and what were their differences?
While I'm focusing on the Intel 8080, I've also seen information on the Taito and Midway 8080 boards used by some 70s arcade games (Space Invaders, Gun Fight, etc.). I'm interested in learning the different types of boards and chipsets that were produced, and what the main differences between them were (such as register and instructions differences).
Why are the first three instructions of Space Invaders NOPs?
Here are the first four instructions of Space Invaders from the decompiled ASM:
0000 NOP 0001 NOP 0002 NOP 0003 JMP $18d4 # ...
Why would the engineer deliberately leave three NOPs at the beginning of the program?
Although this question might be opinion based, it might start some interesting discussion (it provides space for a jump instruction for debugging). There's a semi related question Why use static RAM addresses instead of the stack? that has been well received already.
What are the Intel 8080 Instruction at OP Code 0x20 and 0x30?
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) instruction.
However, only the 8085 has the RST5.5, RST6.5, and RST7.5 hardware interrupts. Are the instructions at 0x20 and 0x30 just NOP on 8080 hardware?
While programming-related, answers to this question could contain a detailed description of the 8080/8085 hardware, and explain that the 8085 had additional instructions for interfacing with a serial port or set interrupt mask, which were only present on the 8085 and not the 8080.
The last two questions would also be a chance to share some of my knowledge Q&A style, while also gathering answers from those who have more experience with the hardware itself.