8085 Software

This covers creation of software support for the 8085 CPU within the framework of the z88dk and also with MS Basic 4.7. Specifically, the 8085 undocumented instructions will be covered, and some usage possibilities provided.

Future work is to build a re-entrant IEEE floating point library specifically using the stack relative instructions found in the 8085 undocumented instructions.

8085 Microsoft Basic 4.7

The Microsoft Basic 4.7 source code is available from the NASCOM machine. Although the NASCOM machine was a Z80 machine there were only minor changes to the original Microsoft Basic 8080 code. Therefore it is an ideal source to use to build a 8085 Basic System.

At this repository the 8085 RC2014 Microsoft Basic is being developed. Currently fully working with the RC2014 ACIA Serial Module (from the RC2014 Classic ][). Some initial performance testing has been done, and there is little difference (< 1%) vs. the Z80 at the same frequency.

A version of Microsoft Basic 4.7 for the 8085 CPU Module together with the Am9511A APU Module has been built, as well. This version adds the full performance of a hardware APU to the 8085 CPU providing the “complete performance package”.

Z88DK Support

Support for the 8085 processor is available from the z88dk. The sccz80 C Compiler, combined with the classic library and z88dk-z80asm assembler provide the necessary components.

Support for the 8085 CPU Module for the RC2014 has been provided using the underlying MS Basic as a program loader and debugging tool. This is reached through the rc2014 target basic85 subtype. This uses the standard RST serial interfaces (provided by MS Basic) and the HLOAD keyword to upload code compiled for $9000 origin (by default). Compiled programs can use any memory from $8400 through to $FFFF.

Also a rc2014 target ROM subtype acia85 has been provided to allow on-the-metal embedded applications to be written. The full 32kB of ROM and 32kB RAM is then available, with the option to toggle out the ROM if needed for CP/M or similar systems.

Within z88dk the mbf32 math package has been optimised to support 8085 undocumented instructions.

The z88dk sccz80 C compiler is used for 8080, 8085 and Gameboy Z80 CPUs. This compiler is supported by the z88dk classic library. Over the past few weeks, I’ve reworked all of the sccz80 compiler support primitives (called l_ functions) to make them reentrant, and to optimise them for the respective CPU.

8085 Undocumented Instructions

Over the years since launch several very useful undocumented instructions designed into the 8085 have been found. These instructions are particularly useful for building stack relative code, such as required for high level languages or reentrant functions. However, perhaps because of corporate politics, these useful instructions were never announced, and thus were never widely implemented.

There is a reference to these instructions and their use in Intel mnemonics, but I prefer to use Zilog mnemonics. So I’ve modified the CLR table to support the 8085.

The z88dk z80asm assembler provides a few macro instructions (although it is not a macro assembler) to simplify programming. These instructions are usually a useful sequence of normal instructions that can be issued with no side effects (eg. setting flags) that may streamline combined 8085 / z80 programming.

Discussion on the Instructions

Some things to think about (and then do).

  • Use the Underflow Indicator (K or UI) flag with 16 bit decrement and JP KJP NK instructions to manage loops, like LDIR emulation, more cleanly. 16 bit decrement overflow flag K is set on -1, not on 0, so pre-decrement loop counter.
  • Use the LD DE,SP+n instruction with LD HL,(DE) to grab from and LD (DE),HL to store parameters on the stack. Can use this with a math library to make it reentrant, for example, and also relieves pressure on the small number of registers.
  • Use the LD DE,SP+n instruction with LD SP,HL to quickly set up the stack frame. For example LD HL,SP+n, DEC H, LD SP,HL to establish 256-n stack frame.
  • Use RL DE together with EX DE,HL to rotate 32 bit fields.
  • Use RL DE together with ADD HL,HL to shift 32 bit fields.
  • Use RL DE as ADD DE,DE to offset into tables and structures.
  • Use SUB HL,BC for 16 bit subtraction.
  • Remember EX (SP),HL provides another “16-bit register”, if SP+2 is the location of the return, and SP+4 is the location of first variable.
  • Learn how signed arithmetic can be improved using the K flag.

Since we know that the 8085 undocumented opcodes are available in every 8085 device they can be relied upon for any 8085 system. The challenge will be to take existing 8080 programs, such as Microsoft Basic and CP/M, and implement improvements using these 8085 specific instructions.

In reworking the z88dk sccz80 l_ primitives to make them reentrant and to optimise them for the 8085 CPU, I have found the LD DE,SP+n instruction very important. Using this instruction it is possible to use the stack as effectively as static variable storage locations. The alternative available on the 8080 (and Z80) LD HL,N , ADD HL,SP takes 21 cycles, and clears the Carry flag. With the few registers available on the 8080 losing the Carry flag to provide state causes further cycle expense, spared with the 8085 alternative.

To load a single stack byte using LD DE,SP+n , LD A,(DE) is only 4 cycles slower than loading a static byte using LD A,(**). Also, loading a stack word using LD DE,SP+n , LD HL,(DE) is only 4 cycles slower than loading a static word using LD HL,(**). Given that variables can be used in-situ from the stack or pushed onto the stack from registers rather than requiring the overhead of the value being previously loaded into the static location, this small overhead translates into about 3 stack accesses for free compared to static variables.

One small design oversight in the Program Status Word of the 8085 is however quite annoying. The flags register contains a single bit that always reads as 0. A $FFFF pushed to AF is read back as $FF7F. This means that unlike in the Z80, it is not possible to use a POP AF , PUSH AF pair as a temporary stack store, which invalidates AF as one of the only 3 additional 16-bit registers as an option, making things even tighter when juggling the stack. I’d call it annoying AF.

The RL DE and SUB HL,BC instructions are very useful to build 16-bit multiply and divide routines effectively. They have contributed to useful optimisations of these primitives. The saving in bytes over equivalent 8080 implementations has allowed for partial loop unrolling, which also speeds up the routines by reducing loop overhead. Initially, I was concerned that the SUB HL,BC function didn’t include the Carry flag. But in hindsight it is not possible to effectively carry into the registers, and using the 8 bit SUB A,C , SBC A,B instructions via the A register is the way to manage long arithmetic.

8085 CPU on the Z80 Bus

For RetroChallenge 2021/10 Updates scroll down.

The 8080 CPU stands at the root of microprocessor development over the past 50 years. Although it was the first commercially successful device, it was followed quickly by two different processors with different bus characteristics. This is a record of interfacing one of the descendants, the Intel 8085, with peripherals and modules designed for the other descendant, the Zilog Z80.

All three of these devices, the 8080, the 8085, and the Z80 were implemented with 40-pin DIP packaging, which limited the number of pins they could use for bus signalling. The 8080, requiring 3 power supply voltages, was particularly limited as it didn’t multiplex the address or data lines, but rather needed to share the data lines for status information. More about the 8080 can be read at Wikipedia, or CPU Shack. I will not add to it here.

Derived from the 8080 and implemented by the same lead designers and architects, the Zilog Z80 uses four lines to signal general timing on the bus. In addition, a M1 line is used to signal that an interrupt is being processed and that an interrupting peripheral needs to provide an address (or vector) to which the CPU should jump in IM2 mode.

The Z80 rationalised the power requirements down to +5V and GND, which allowed a simpler and more explicit set of bus controls to be provided. As the Z80 implemented two address spaces, one for memory and one for Input/Output ports, it was useful to have two separate lines signalling memory access and Input/Output access. In this way a peripheral only needed to handle one of the two signals, depending on whether it was memory or a I/O address space peripheral device.

In addition the Z80 has two lines providing signalling for Read or Write. The timing was designed so that the data on the 8 data lines was valid at the point when the respective signal was deasserted. The Z80 would hold data it wanted to write or output until the write signal was deasserted, and it would latch and read the bus when reading or inputting data when the read signal was deasserted.

Z80 I/O Cycle Timing

With only minor differences, the Memory and Input/Output lines are operated with similar timing, and this is aligned mostly with the Read and Write signals. This enabled system designers to build very simple bus interfacing for their Z80 based systems.

There are many additional features and alternatives here, around Interrupt Mode 2, timing for sampling the Ready pin which causes the Z80 to pause, and other minor timing issues. However, they are not relevant for most purposes.

Most system designers used these four signals to create memory write, memory read, I/O write and I/O read signals. Then one signal line, together with a chip-select generated by the address lines (directly in simple systems, or through logic in more complex systems) was enough to operate each component of the system.

For the 8085, the Intel architects took the bus interface in another direction. They integrated several components from the support chips for the 8080 into the silicon die, and produced new features which made the 8085 much more useful as a micro-controller than the Z80.  For the bus, the major change was to multiplex the data lines with the low address lines. This step allowed them to reuse the 8 saved lines on the 40-pin DIP for other purposes.

Multiplexing the address and data lines meant that they had to add an external address latch, to capture the lower address values, before either writing data or reading data from the bus. The normal read and write lines are present and they behave in a similar manner to the Z80.

8085 Micro Architecture – Showing external decoder

In a significantly different solution to the Z80, the 8085 uses only one line to differentiate Input/Output and Memory addresses. Using the sense of the line high or low to indicate whether the I/O address space or the memory address space is being addressed. The timing on this IO/M line is also substantially different to the Z80, where here it is valid for the entire cycle of an  instruction. It does not become valid when the bus address is valid, rather it is valid from the start of the instruction through to the completion of the instruction.

8085 General Cycle Timing

This is the first significant divergence from the Z80 system bus, and it causes issues with peripherals that require an enabling signal to be provided after the address lines are stable. In most designs a decoder was required to produce signals for attached peripherals.

Generating Z80 /IORQ and /MREQ from 8085 signals

As many Z80 standard peripherals and also Motorola peripherals need to have the /IORQ line valid when the address is stable, we need to generate a Z80 compatible /IORQ (and /MREQ) signal. There are textbook “decoder” circuits available to produce the four system signals /IOR /IOW, /MEMR and /MEMW from the 8085 IO/M signal and /RD, /WR, but there is no standard solution for using the 8085 on the Z80 bus. This problem we are going to solve.

From the Z80 datasheet the /IRQ and /MREQ signals are almost exactly tied to the timing of the /RD and /WR signals. Therefore we can use /RD and /WR with some combinational logic to produce mostly correct timing for /IORQ and /MREQ. We need to have a valid signal when either /RD or /WR is low (active). If both are high, then the result should be also high (inactive). Both /RD and /WR are never active, but for convenience we can let the result be active if both are. In positive logic this would be generated by an OR gate. But with inverted logic (active low) this is implemented as an AND gate.

/RD/WRResult – /RD./WR
000 – Invalid state.
010
100
111
Intermediate Truth Table

To generate the /MREQ signal we are looking for the time when IO/M is low whilst either /RD or /WR is low. In negative logic this is an OR gate, where the signal remains high unless both /MREQ and /RD or /WR are low. So to generate /MREQ we need to provide ( /RD AND /RW ) OR IO/M.

/RD./WRIO/M Z80 /MREQ
000 – Only when both are active.
011
101
111

To generate the /IORQ  signal we can recognise that it is simply the same /RD /WR logic but the IO/M line needs to be inverted or NOT converted. So we can generate /IORQ by ( /RD AND /WR ) OR NOT IO/M.

From this solution we can simplify the expression into either NAND or NOR gates. Taking NAND gates as the basis the solution can be simplified into 4 gates that can fit into a 7400 device.

Other Bus Timing Issues

Several Z80 peripherals use the READY signal to cause the Z80 to wait until they are ready to read data from the bus, or to write data onto the bus. The Z80 implements one wait state whenever it uses I/O instructions, to enable slow peripherals sufficient time to signal they are not READY to proceed. The 8085 does not add in the automatic wait state, so there may not be sufficient time for them to signal the CPU to wait. There are standard circuits available to add one wait state into I/O bus cycles.

Motorola bus peripherals use an E or Enable clock to signal that they are being addressed. For the Z80 bus, this is typically implemented by inverting the /IORQ signal. However, for the 8085 using the method above, there may be insufficient time between the E (inverted /IORQ) and stabilisation of the address.

Z80 peripherals capable of Interrupt Mode 2 use the M1 signal to determine when they should place their interrupt address (vector) on the bus. The 8085 does not generate this signal, but since the 8085 does not support IM2 mode anyway this point is probably mute.

8085 CPU Module for RC2014

8085 CPU Module PCBs are available on Tindie. Combine with a Memory Module PCB to save postage.

The RC2014 Bus and Modules have been available now for some time, and the Z80 nature of the system bus provides for simplicity in the system design. There is no buffering or conversion by the CPU Module, and individual peripheral Modules are left to convert bus (or Z80) signals to suit their own requirements.

I have previously designed a few Modules for the RC2014 and, since I’ve now an interest in 8085 processors, I thought that it would be a good time to design a 8085 CPU Module.

In researching the requirements for a 8085 CPU Module to work with the RC2014 Z80 bus and standard peripheral Modules, I found the Glitchworks 8085 SBC and also Alan Cox’s 8085 designs. My initial design replicated the bus interface signalling of these two designs.

After building the first version of the 8085 CPU Module I found that the Motorola 68B50 ACIA based RC2014 Serial Module didn’t work properly. This is because on the module the required E clock is derived from Z80 /IORQ timing, and the simple method of inverting IO/M as /IORQ doesn’t provide the timing needed. The 68B50 requires the bus address to be stable before E (or /IORQ inverted) is asserted.

A second version of the 8085 CPU Module was implemented, using the above method for generating the /IORQ and /MREQ signals.

8085 CPU Module

Initial tests using the modified Microsoft BASIC 4.7 for 8085 used with the RC2014 have proven to be successful. The 8085 running BASIC is marginally slower than the Z80, but it is less than 1%.

8085 Module assembled with OKI CPU

Further analysis on the performance of AHCT buffers vs unbuffered I/O to come.

Please read further for software support, including undocumented instruction discussions.

RetroChallenge 2021/10 – Am9511A APU Support

The current hardware doesn’t supply a wait state to the CPU, so the hardware interface to the APU Module designed for RC2014 doesn’t work. The 8085 CPU allows only 25ns to 30ns (depending on the manufacturer specification) for assert not READY (or /WAIT). Am9511A takes 83ns to assert /WAIT.

The retro-challenge is to extend the current 8085 CPU Module design to include a wait state generator for IO instructions to support the APU Module and the UX Module.

1st Update – 2nd October

Getting to Am9511A APU support for the RC2014-8085 machine means firstly getting the fundamental 8085 platform working.

The RC2014 is supported by the “newlib” of Z88DK, which is meant for Z80, Z180, Z80N (Spectrum Next) processors, and the 8085 is supported by the “classic” library. So this is the first time that a newlib machine is using classic lib libraries. Confusing? Yes I find it so.

Anyway the trick is just getting the right pieces to link together. Having ZIF ROM and TL866CS Programmer helps with fast programming cycles.

RC2014 – 8085 CPU Module, APU Module, and Memory Module

2nd Update – 3rd October

Now the z88dk RC2014-8085 ROM build using the ACIA Serial Module is working (along with the RAM build supported by Basic), I’ve spent the past days tidying the ACIA builds around my various repositories, to keep everything consistent. So now my BASIC builds for both 8085 and Z80 are aligned with RC2014 HexLoadr BASICCP/M-IDE ACIA, and also the z88dk ACIA newlib device code. Also took the time to clean up some the SIO device code too.

@suborb is working on the z88dk classic library crt0 and compiler intrinsics, as they’ve been stuck in both classic and newlib and are a bit disorganised. Hopefully the result will be one set that can be used for both compilers (zsdcc, and sccz80) and both libraries, across multiple machines (8080, 8085, GBZ80, Z80, Z180, Z80N, etc) which will make maintenance much easier.

Waiting now for China to come back from National Day holiday, so I can get started with new hardware.

8085 Wait State Generator

3rd Update – 8th October

As noted above the window of opportunity for a 8085 bus peripheral to signal not READY is very short. In fact is is no more than 30ns from fall of the ALE signal, and this is 30ns before the /IORQ signal is even enabled.

8085 Timing showing ALE fall to /READY as tLRY.

Timing information from the 8085 datasheet shows tLRY as maximum 30ns, and tLC as minimum of 60ns.

8085 CPU Timing – compare tLC to tLRY

To be able to connect devices designed for the Z80 bus to the 8085 CPU we will need to implement a wait state generator. In the best case this will only affect I/O cycles, and will not slow down normal memory read and write cycles.

Designing the 8085 /IORQ Wait State Generator

As the need to generate a wait state was well known at the time of release of the 8085, several sources include the information required for the design of a basic solution. It is left to the reader to determine how to use the created wait state though.

For our purposes we need to have a wait state generated only for peripheral devices, accessed using the I/O instructions. Therefore we can modify the above circuit to only generate a wait state when the I/O address space is active, or when the external Z80 bus /WAIT signal is active. The below circuit produces a /READY signal that provide 1 wait state whenever the I/O address space is active, and can continue to produce wait states until the /WAIT signal is de-asserted.

As the static RAM / EEPROM memory devices we are using are not sensitive to the timing of the /MREQ signal, the NAND gates assigned to generate a correct Z80 /MREQ have been recovered and reused in the implementation of the wait state generator. Therefore the revisions required only one additional device on the PCB. Based on this design a revised 8085 CPU Module was created, and ordered. Due to arrive around October 18th, which won’t leave much time to finish before the end of the RetroChallenge. It will be a rush, as usual.

8085 CPU Module – Version 4

5th Update – 13th October

The new 8085 CPU Module PCB arrived, so wasting no time I’ve build one up to test. And it works!

8085 CPU Module - Version 4
8085 CPU Module – Version 4

It is interesting to look at the signals actually appearing on the RC2014 Bus during the operation of the APU. Here we have a floating point read from the APU, 4 bytes, where the wait state generator produces sufficient delay (1 wait state) to allow the APU to generate its own /WAIT signal for the last two bytes.

8085 CPU Module – APU Read Cycle

The floating point write cycle is similar but the duration of the /WAIT signal from the APU is longer, and the APU needs to assert it on every byte written. Note that tRYH is 0ns, so there is no need to hold the /READY signal beyond the clock rise point.

8085 CPU Module – APU Write Cycle

To support the Am9511A APU Module the /WAIT signal has to be patched to the USER1 Pin (if using the standard RC2014 backplane), which allows the Am9511A to extend the single wait state generated by the 8085 CPU Module for as long as the APU needs.

8085 CPU Module shown together with Am9511A APU Module and Memory Module (160kB).

I’ve prepared a specific version of MS Basic 4.7 for the 8085 CPU Module when used with the Am9511A APU Module. Initial testing is working. It is looking very good to achieve the RetroChallenge goals. Please read further at the 8085 Software post for more information.

With the Wait State generator functioning, it is now possible to use the UX Module for a VGA screen and PS/2 Keyboard.

Stand-alone 8085+Am9511 RC2014 System.

6th (Final) Update – 26th October

Rework of z88dk classic 8080/8085/gbz80 library l_ functions.

When working with the 8085, the biggest issue is the continual pressure on the few CPU registers. Alongside the 8-bit accumulator a register and the 16-bit accumulator hl registers we have only two additional register pairs that can be used, the bc and de registers. This gives the system programmer few options but to use static memory locations to store intermediate values, which leads to non-reentrant code.

Having non-reentrant code is normally not a problem, but it does lead to issues when multiple threads (or tasks) are trying to use the CPU at the same time, for example when a multi-tasking operating system is to be supported. So it is useful to try to build reentrant functions that use the stack for storage of intermediate values, rather than static memory locations.

The designers of the 8085 had this in mind when they designed the additional functions found the 8085 silicon. The “new” instructions make it very efficient to build stack relative functions (compared to the 8080), and this relieves some pressure on the small number of registers.

However, there was one oversight made by the designers, as the 8085 af register pair cannot be used, in contrast to the z80, to pop and push arbitrary words on the stack. This reduces the number of available 16-bit registers by 1 of possible 4. There is one flag bit that always reads as 0, which is an subtle but annoying limitation of the 8085.

For the past two weeks I’ve been working on a refresh of all of the integer and long basic compiler use l_ functions to try to make them reentrant and, where possible, to optimise them for the 8085 CPU. I’ve also moved the improvements to the 8080 and Gameboy Z80 CPU where possible too.

As background, some of these functions originate from the 1980s and 1990s in the Amsterdam Compiler Kit, and haven’t been updated or improved for the past 20 years. They weren’t broken. But they were in need of some attention.

So this update is the final one in the October 2021 RetroChallenge. All the new functions are checked in and are now part of the z88dk.

8085 CPU based ISSF Target Turner

My club uses pneumatic systems to turn the ISSF Targets, which are controlled by a timing system. One of the members asked me to help build a phone interface for the systems.

The systems are used for many courses of fire, and there are quite a few options to manage. On the front panel there is a RESET, which is tied to the CPU RESET, and a FACE button which returns the targets to face the shooter for scoring.

Target Turner Front Panel.

It turns out that the retired systems are based on a 8085 CPU, in the classic minimum configuration with an 8155 providing 256 Bytes of RAM, and input and output ports. There is a 2732 UV PROM holding the program.

CPU Board for the Target Turner.

So, how do we get these devices online? My thoughts are to add a serial port so that the system can be controlled remotely, then to use an additional WiFi enabled device which can present a web interface to the Range Officer to control proceedings.

Existing ROM

First step is to see what is going on under the hood here. So using the TL-866 the binary code on the ROM was read, and then using z88dk-dis the existing code could be interpreted.

It was interesting to see a very simple method of operation in the existing ROM. The system can only change course of fire if it is RESET, when it reads the position of the switches, and then halts awaiting an interrupt to trigger the course of fire. When the string is finished it will return to repeat the same course of fire.

Timing was based on a delay circuit providing 500ms of delay per unit. Perhaps it is not 100% accurate, but good enough for the application.

I believe that I found a bug that has been latent in the device for the last 40 years. It seems that an address byte was reversed, which would cause a jump into empty addresses. Not sure why no one realised that previously.

Building Serial Interface

I’m planning to build a simple serial interface which will read a character, and then change the course of fire based on that character. Initialising the course of fire can be then done by the web interface, by triggering an interrupt, or by using the wired front panel interface.

After asking the experts I learned that the SID/SOD pins on the 8085 can be used as a bit-bang serial port. In fact that is the standard way of building a serial port for early systems. The code for building serial transmission is included in the early application notes.

The serial code works perfectly at 9600 baud on this 3MHz system. Since only one character will be received and a few transmitted on boot, there are no performance issues to consider.

I’ve written the upgrade code to replicate the front panel selection process, and to allow the system to behave exactly as before when no serial input is available. When a serial command is available, which is triggered by activity on the RST6.5 line, then the system will set a different course of fire than is shown on the front panel. The string can be triggered either by the front panel, or by the interrupt related to the serial interface.

ESP-32 Web Interface

Following a bit of a search the Adafruit HUZZAH32 Breakout presented itself as the best solution to web enable the Target Turner. It can be powered by 5V, and the RX is protected against 5V input by a diode.

The physical interface is going to be a FTDI Basic style connector. Using this connector will allow me to best the 8085 first, and then build the web interface and test separately from the Target Turner. The last step will be to integrate the two devices into a system.

Using the simple serial character interface, it should be possible to present an active web page to the Range Officer.

There are many, eg, tutorials on how to build active web pages using the ESP-32 and WebSockets.

More when this is progressed further.