Consoles
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Computers
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Sinclair ZX Spectrum |
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Commodore Amiga series (A500/A1200/etc) |
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Chips
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Zilog Z80 |
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Motorola M68705 |
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NEC V60/V70 |
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Intel i960 |
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Arcade machines
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Theory
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Tools & Utilities
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ROM hacking |
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Ah, what can be said about ZiLOG's amazing Z80 chip. The most popular chip for video games, during the 8-bit era, used in many different systems. It's amazing cheapness, simplicity and elegance ensured it would remain in constant use throughout the 1980s and still in some systems in the 1990s.
This unit powered many arcade games, such as Pacman, Mr Do, Galaxian, etc. It was also the brain behind the Sega Master System, the ZX Spectrum, the MSX, and many more. Towards the end of it's life cycle, it was relegated to the role of a sound CPU in many machines, such as the NeoGeo, the Sega Genesis, etc.
Here's the register set it used -
| Register | Description |
| AF | Accumulator & Flags |
| BC | Counter |
| HL | General purpose |
| DE | |
| IX | Indexing |
| IY | |
| PC | Program counter |
| SP |
Stack pointer |
| IR | Interrupt & Refresh |
| IFF1 | Interrupt flip-flop 1 |
| IFF2 | Interrupt flip-flop 2 |
| IM | Interrupt mode |
All these registers are 16-bit. The AF/BC/HL/DE registers could be accessed as either a 16-bit register, or two 8-bit registers, as in the Intel x86 series. In fact, many of the Z80's features were shared with the Intel range. The IR register was only viewable as two 8-bit registers, the I and R registers.
There are also the prime registers (AF', BC', DE', HL'), which are kind of like a secondary set of registers. They can be switched with their normal equivalents at any time.
Upon power-up, the Z80 would clear all the registers to zero, except for the IX/IY registers which would be 0xFFFF. It would then start to read an opcode from the current PC, decode it, and execute it. This would repeat until it was powered down.
The Z80 had a 64Kb memory space, as well as a 64Kb I/O address space. The memory space was accessed using the normal instructions as well as being used for opcode-fetching, but the I/O space could only be accessed using special I/O instructions. Many systems only decoded the low 8-bits of the I/O addresses, to provide a 256 byte I/O space.
There are two types of Z80 - the Z80A and the Z80B. As far as I am aware, the only difference is that the B model can go faster. Anyone know anything about this?
Resources
- I'll say this now - go see Sean Young's Z80 page on MSXnet! This man has done far too much research into the Z80, and there's no way I could possibly try and compete ;-)
- Pin descriptions, for the hardware guys.
- Description of the flags register.
- The instruction set, which lists all opcodes etc.
- Go get some software emulators to run proggies on.
- Links to various Z80 sites.
- And don't forget to grab a disassembler from the ROM hacking page!
Pin descriptions
Based from the excellent
book "Understanding Your Spectrum", By Dr. Ian Logan (ISBN 0 86161 111
X, but probably out-of-print anyway...) -
The Z80 is a silicon chip with forty pins numbered from 1 to 40. The above diagram shows the pin arrangement of the Z80, and the names associated with each pin. The lines labelled +5v, GND, Clock, and the Dx/Ax bus lines, are active high. All the rest are active-low.
- +5v - the power line. A +5v supply is required by the Z80 microprocessor.
- GND - the ground line.
- Clock - the clock input, usually around the 4MHz range.
- D0-D7 - these eight lines form the data bus that carries data bytes to and from the processor to the memory.
- A0-A15 - these sixteen lines form the address bus that carries address from the processor to the memory.
- RD - the read line. This will be active whenever a byte of data needs to be read from memory.
- WR - the write line. This will be active whenever a byte of data needs to be written to memory.
- MREQ - the memory request line. This line is activated whenever a byte of data is being passed to, or from, the processor.
- RFSH - the refresh line. This line is used to refresh dynamic memories.
- M1 - the memory fetch line. This can be raised during a read operation, to indicate that the byte being fetched is an instruction. As far as I can tell, this is generally only used for encryption purposes.
- IORQ - the input/output line. This line is used instead of MREQ, when an IN/OUT instruction wants to access the I/O address space.
- HALT - the halt line. This line is active only while the HALT machine code instruction is being executed.
- BUSRQ - the bus request line. The Z80 allows external devices to temporarily stop the Z80, so that they can have access to the buses/memory/etc. A device asserts this line, then waits for the Z80 to respond with the BUSAK signal.
- BUSAK - the bus acknowledge line. Raised when the processor has given the go ahead for a device to take control of the bus.
- RESET - This line is used to initialise the microprocessor. It is therefore activated as soon as the power is connected to the machine.
- WAIT - the wait line. A slow memory chip may require extra time in order to complete a read/write operation. It does this by signalling the Z80 via this line. Some systems like the Spectrum's ULA chip use this to allow sharing of RAM between two processors.
- NMI - the non-maskable interrupt line. This line, when active, leads to the microprocessor stopping the execution of the current program. Instead, the processor executes a special interrupt routine. This line is rising-edge triggered (probably), so that a NMI won't be repeatedly handled.
- INT - the maskable interrupt line. This works like the NMI line, except can be disabled by software. Also, this is level-triggered, so the Z80 needs to disable interrupts as soon as it receives one, so that if the line remains active it won't cause multiple interrupts.
To do a complete read cycle, for example, the Z80 would first set the address lines to the required address, then assert the MREQ and RD lines to indicate that the data should be supplied. The memory hardware will respond to the MREQ line, and will supply the byte via the data lines.
The Flag register
| Bit | Initial | Name | Description |
| 7 | S | Sign | Set when the result was negative. |
| 6 | Z | Zero | Set when the result was zero. |
| 5 | 5 | Bit 5 (undocumented) | Usually a copy of bit 5 of the result. |
| 4 | H | Half-carry | Set when a carry occurred between bit 3/4 of the result (used for BCD). |
| 3 | 3 | Bit 3 (undocumented) | Usually a copy of bit 3 of the result. |
| 2 | P or V | Parity/Overflow | Set to indicate either even parity in the result, or a 2's complement sign overflow. |
| 1 | N | Negative | Set when the previous instruction was a subtraction, clear when it was an addition (used for BCD) |
| 0 | C | Carry | Set when a standard carry occurred. |
Sometimes the P/V flag is called P or V, depending on whether parity or overflow is meant. It depends on what sort of instruction you've just executed as to what you should treat it as.
The instruction set
This is a complete list of all the opcodes for the Z80, both documented and undocumented. This was compiled by the Z80 guru Sean Young, and is also available on his Z80 page.
Click here to read it.
The Z80 is infamous for having a large number of undocumented instructions, which ZiLOG probably never intended to be used. However, a lot of people did find out what they were, and made the most out of them. So, any Z80 emulator worth it's salt should be able to handle them. This file, again from Sean Young's site, details them all.
Z80 emulators
This is a list of the major software emulators available for various platforms. Some are Intel only, some are in C.
- RAZE. I'm a bit biased towards this one (seeing as I wrote it ;-), but it is probably the best Z80 emulator available. It only runs on the Intel x86 platform though, but is quite fast, has a clean interface, and features almost perfect emulation. Also has proper IRQ line handling, unlike most others.
- Marat Fayzullin's emulator is one of the most famous ones out there, and is used in many projects. It is quite slow though, and has a number of bugs. Written in portable C.
- Marcel de Kogel's. Based off Marat's one, but has an optional x86 component to make it slightly faster on that platform. Was used in MAME till they wrote their own. I prefer this to Marat's. Written in portable C, with optional x86 assembly.
- MAME. The MAME Z80 core is quite good, being fast and portable. It comes in two flavours, a C version and a Intel x86 version. However, it is extremely hard to extract it from the rest of the MAME source code, and also any code using it will probably fall under the MAME license...
- MZ80 is Neil Bradley's effort. It is very fast, being written in x86 assembler. However, it does have a lot of bugs, can't really handle bank switching too well and IMHO the interrupt handling is a bit dodgy. Some people seem to like it though, so give it a try ;-)
- MAZE was written by Ishmair, who now seems to have left the scene, so don't expect any updates here. This emulator is quite good, a bit like MZ80. At the time, this was *the* emulator to use, but now MZ80 is a better bet. x86 asm only.
Links 'n' stuff
Go see Sean Young's page! Now!
There's always the ZiLOG page, but there's not much there...
There's a *lot* of stuff on the Official Z80 Support Page. Pinouts, hardware stuff, software stuff, etc. Far too much stuff in fact :-)
The Spectrum FAQ has lots of Z80 tech stuff in too.
And lastly, the most important thing for anyone writing a Z80 emulator. There is a Z80 emulator called YAZE, by Frank Cringle, which comes with a CP/M program called ZEX. This little proggie will test your Z80 emulator and see how accurate it is! Definitely worth using...