Writing Simple Routines

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	Whilst I never intended to write a page detailing how to write simple / basic software routines, I seem to be being asked to do this so many times, I guess I�ve bowed to pressure in the end. However, there are STILL great problems in doing this so as to cover as many processor devices as possible. Thus I�ve drawn up a couple of simple rules for myself. Firstly, I am not proposing to use any specific Assembler mnemonics so as not to confuse users of another processor. Mnemonics can be particularly annoying with processors using the same HEX coding AND instructions, yet the mnemonics are different for each device e.g.

Z80	8080
LD H,VALUE	LX1 H,VALUE
LD A,C	MOV A,C
DEC B	DCR B


	All three of these codes are common to both of these processors and they both use the same OP codes too! Users familiar with the 8085 processor that ALSO uses these same codes will notice that it uses the 8080 mnemonics rather than those belonging to the Z80...

Another ruling I set myself was to try NOT to use special multi-operation code specific to one processor, like the LDIR, LDDR, CPIR AND CPDR Z80 codes. These are very useful routines that perform multiple operations with the programmer simply entering values into certain specified registers and pressing the �Go� button. Really like a sub-routine already built into the processor.

My last rule was to avoid �tidy� and elegant� pieces of coding that use �clever� techniques to achieve the required goal. Unfortunately they generally involve a measure of �Nellie knowledge� and therefore might not be well understood by someone new to the subject; thus all the coding here is SIMPLE to the extreme - BUT will enable the micro project builder to write his or her code that will work! There�ll be time to tidy up the code and make it look more �professional� at a later date.


	I /O routines are the bread and butter of all micro code as without it your program cannot inform you of the results or accept your input. Some processors treat any I/O as memory, whereas other devices give the I/O it�s own special routines and even timing. At the end of the day there is not a lot of difference to the software writing as you simply pick the appropriate instruction. If we use a couple of the site projects as examples here, this will be better explained. Note that both the EEPROM programmer (Z80 based) and the AT Keyboard interface (6809 based)


	use the same DUAL serial comms chip, namely the Zilog 8530 SCC. Thus you will be able to see the same commands needed to be sent by both processors; the Z80 with it�s specific I/O ones and the 6809 ones that call everything �memory�.

Sample of code used to begin initialising the 8530 SCC chip:

Z80 HEX	does what?	6809 HEX	does what?	what are we attempting to achieve by this?
3E	LOAD A REG	86h	LOAD A REG	The 8530 needs two instructions to allow the processor to talk to
09	WITH 09h	09h	WITH 09h	any of it�s many registers. Here we tell it we want access to Reg 9
D3	SEND TO	B7h	SEND TO	We send this register �request� to the address of the control regs.
21	I/O 21h	80h	ADDRESS	for the �A� half of the SCC. Note that the Z80 uses a specific I/O
3E	LOAD A REG	01h	8001h	address, in this case 21h, whereas the 6809 thinks of it as memory.
C0	WITH C0H	86h	LOAD A REG	This is the second part of the instruction where we now send the
D3	SEND TO	C0h	WITH C0H	value we want to send to Register 9. This instruction is a Reset.
21	I/O 21h	B7h	SEND TO	Note that the instruction is sent to the same address as before. The
		80h	ADDRESS	internal workings of the SCC will have directed the value where
		01h	8001h	we wish it to go.


	Hopefully all will have been able to understand the above instruction. To use one half of the dual SCC device, one has to set up a number of different registers with different values for baud rate, data bits, stops bits, interrupts etc. All are entered in the same way as above and in the case of the Z80 use up a total of 56 (decimal) bytes - and this doesn�t include the final instructions to either ENABLE the receiver or transmitter!


		MULTIPLE BYTES TO BE SENT..

	There will be occasions when there are several bytes to be sent at once. In the example above we sent each one to the same address, but in the case of others like the displays to the left, each digit will have it�s OWN specific location. Now it�s up the programmer to decide when the time is right to .


	�optimise� the code rather than simply repeating the same code over and over again until the job is done. In the case of displays like these with built in driver chips, it is customary for the device to have a pair of address lines to take to the Address Bus. (4 digits will need only 2 address lines to select any one of them). In the example to the left we are using 2 x displays (8 digits) so how you connect them �hardware-wise� will determin how you address them in the software. In the case of the Site Programmer, I took both of their A0�s and A1�s to appropriate address connections, and then took each chip select (/CE) line to a different output of the 74LS139 decoder chip I�d designed into the circuit - The right hand one to output Q0 and the left hand one to Q1. With Address Bus lines A4 and A5 taken to the �139, this meant that the far right digit would be addressed as 00h with the next to the left being 01h, 02h, 03h, with the fifth digit (the far right of the SECOND display) being addressed as 10h, with the next as 11h, 12h and lastly 13h for the far left. Confusing? Indeed! Take a few minutes to work it out yourself and it�ll all come clear.

Whilst I�m afraid that it would be of little help to show the display load routine from the EEprom Programmer program as it incorporates the OUTI instruction which is unique to the Z80, so for this example I will show that used in the 8085 project - which will of course work equally well on the Z80 or the 8080. In keeping with my liking for retaining similarities in my designs, the addressing for the displays in both the Programmer and the 8085 Site Project are identical. Note that the �HL� pair is simply two 8 bit registers combined to form one single 16 bit one. Most processors have at least one that can perform this operation. In the case of the 6809, there are two indexed registers (the X and The Y) which may be used in a similar manner. Although users of the 6502 only have two single byte (8 bit) index registers, these have full increment, decrement and compare facilities, and may be used to transfer messages etc. if the lowest part, or the �Zero page� area of the memory is used. Essentially, it�s a case of getting to know and using to the full, the instructions that come with the micro you are planning on using.

First and foremost, before this Display SUBROUTINE is run, the physical ADDRESS of the first byte of the 4 digit word to be outputted to the display has to be loaded into the HL register pair. The HL pair will then point to a 16 bit address in ROM or RAM, and place the contents into the ACCUMULATOR.

In the case of these processors, namely the 8080, 8085 and the Z80, the op-code is a 21h with two further digits to tell it what immediate values to load into registers H and L. i.e:

21h	LOAD HL PAIR WITH
68h	68h (address low)	Note that this series of micro,
00h	00h (address high)	puts the LSW (the 68) FIRST

If you are feeling really keen :) you can check the example above with the 8085 site project program listing at 0039/003A/003Bh

IMPORTANT!

Note that this program subroutine with it�s four digits is a border line case where it is questionable whether it would have been better to tidy up the code and use another register or memory location to keep track of the passes, thus making it only necessary to write the �Load/Send/Increment� bit only once.Obviously in the case of eight or more consecutive digits one would try not to write out the same piece of coding eight times over!

Subroutine to display 4 x digits in one pass

7Eh	LOAD Accumulator with where HL is pointing (0068h here)
D3h	SEND TO I/O ADDRESS (display far LH digit)
13h	13h
23h	INCREMENT HL (from this example�s 0068h-0069h)
7Eh	LOAD Accumulator with where HL is pointing (0069h here)
D3h	SEND TO I/O ADDRESS
12h	12h
23h	INCREMENT HL (from this example�s 0069h-006Ah)
7Eh	LOAD Accumulator with where HL is pointing (006Ah here)
D3h	SEND TO I/O ADDRESS
11h	11h
23h	INCREMENT HL (from this example�s 006Ah-006Bh)
7Eh	LOAD Accumulator with where HL is pointing (006Bh here)
D3h	SEND TO I/O ADDRESS (display far RH digit)
10h	10h
C9h	RETURN FROM SUBROUTINE


	Of course the moral to any sort of programming at this level is to be sure that what you are about to use does what you think it will. If you are at all unsure then delve deeper and find out - or alternatively don�t use it for the time being, and simply go for a more tried and tested solution, even though it may well use up a few more bytes of code, until you have the time to experiment with the �unknown�.


	Time for a quick word of wisdom. ALWAYS use the correct software manual with the correct micro. Even some that look very similar might not be - and don�t assume either! One can never be sure that a particular instruction with one micro will affect the same condition bits as with another!! Been there, done that... your talking to a professional here... :)


	KEYPADS AND SWITCHES

I can�t remember how many times I�ve been asked for advice on interfacing even the simplest of switches into a micro. I shall therefore go into the basics, which should be transportable to more complex arrangements. This time let�s use the 6809 as an example with a simple inport arrangement with two switches to the outside world, which can easily be expanded up to a total of EIGHT without any hardware changes - except for the extra switches of course!

HARDWARE

With simple 68xx and 65xx systems, a nice and convenient I/O and memory decoding arrangement is by using a 74LS138 decoder chip to divide up the 64K addressing space into 8 x 8K chunks. That�s 8K for a 2764 ROM, 8K for a RAM, leaving a total of 6 more possible addresses without any further hardware. If the �138 is connected like described, then the three address lines from the chip will need to go to A13, A14 and A15. In addressing terms, this means that the ROM is at the top where it has to be, i.e. E000h to FFFFh. The RAM might as well be connected to 138 pin 9 to give it the space from C000h to DFFFh (assuming that it�s a full 8K of course), thus leaving start addresses from: 0000h, 2000h, 4000h, 6000h, 8000h and A000h. So which to use? OK lets go for one in the middle; 4000h. Remember that this address can be either used to read or write - or both, so you could theoretically connect a display to this address AND a 8 bit keypad. FYI, the 6809 PC AT Keypad decoder successfully used the simple 74LS138 arrangement.


	AND NOW FOR THE SOFTWARE...

The simplest way to connect switches into a micro is through an octal driver like the 74LS244 as depicted above. Why not the LS240, LS241, LS245, the smaller LS125, or even the 8155 / 8255 parallel device? (or similar) No reason other than the fact that I have shed loads of the 244�s! As for the 8155/8255, if you are going to need more than one 8 bit port or wish to use some of the other options it offers then go for it. It�s very easy to program and easy to obtain. However, the 244 is a very useful device where just one simple 8 bit port is needed, as in our example here.

Assuming that our 244 is connected to the data bus with it�s enables on pins 1 and 19 connected to pin 13 of the 138 (to enable it at address 4000h), all the inputs (2, 4, 6, 8, 11, 13, 15 and 17) should be tied high with separate pull-up resistors (say, 1 - 2.2K) then connect the push switches between our chosen two inputs, pins 2 and 4, and ground.

To READ in the switches in a simple application like this you POLL them periodically to see if either of them have been pressed down. To do this, first of all you have to decide whether the processor cannot do anything until one or more of the switches is pressed. This routine will therefore be an endless loop. The short routine below shows an example of this:

To test for a condition on the second switch connected to Data Bus bit 1 (thru the 244) the AND WITH value should be 02h. Where you have a succession of buttons or switches to test, it will obviously make more programming sense if you were to AND with FFh, COMPLEMENT the value in the accumulator (which makes every �1� a �0�and visa versa) then check if the whole word is zero. If it is NOT, then you know that a button is pressed and a full scan of the buttons can be actioned. Of course if you want to make things interesting and check for a particular combination of keys where you have several in the bank, then all you do is do a COMPARE with an immediate value that applies to those keys being pressed. For our two keys in this example, the value to compare with would be FCh only when both are down together.

Endless loop switch poll

6809 HEX	means?	What we�re trying to achieve here:
B6h	READ IN FROM:	This is a simple memory read from location
40h	40h	4000h. Note that with our simple example, the
00h	00h	value could be anything from 4000-5FFFh
84h	AND WITH:	Logical AND the result in the Accumulator with:
01h	01h	01h. This will only give a 1 if switch 1 is UP
26h	JUMP if NOT zero	6809 conditional Branch, will jump relative by:
F9h	F9h (relative)	F9h - i.e. BACK6 locations to READ again
......		Condition met and switch down....continue

DELAYS

Yes, the software delay is one of the subjects that comes up most frequently in your Feedback! For that reason I�ll finish off this page with a few useful delay routines. First and foremost one must realise that it is not that easy to predict what a delay is going to be unless one is prepared to sit down and calculate how long each particular instruction takes in MICRO seconds - with a certain value of system clock In all honesty, I never have the time or the patience to do this so I


	generally �cheat� by picking a rough value, then run the program as a loop and adjust the values until it is within the parameters I want to achieve.Here are a few examples of routines I�ve used recently. Remember that a software delay is precisely that; namely a lot of instructions to waste the processor�s time. It is always a good idea to make them SUBROUTINES so that you can accesss them from anywhere within a program - and double them up for longer delays by running two or more of the same sub� one after the other.

34h	PUSH A, B, X, Y
36h
35h	PULL A, B, X, Y
36h
39h	RETURN FROM SUB

How simple can things be sometimes? 6809 routine to give an 18uS delay with a 4MHz crystal.

34h	PUSH A, B
06h
86h	LOAD ACCUMULATOR
10h	WITH 10h
4Ah	DECREMENT ACCUM.
26h	BRANCH NOT ZERO
FDh	BACK TO DECR. ACC.
35h	PULL A, B
06h
39h	RETURN


	Another 6809 routine that may be easily copied for use with other processors. We use the PUSH and PULL instructions to save anything already in the Accumulator that we may wish to use after the delay is finished. By altering the value in A (here we see it set at 10h) you can vary this delay to run it well into the Milli-Second range. Delay for this one is about 115uS from a standard 4MHz crystal again.

F5h	PUSH A,F
3Eh	LOAD ACCUM.
FFh	WITH FFh
3Dh	DECREMENT ACC.
20h	JUMP REL. NOT ZERO
FDh	BACK FD.
F1h	POP A,F
C9h	RETURN


		For your reference, I used a more complex version of the above delay in the AT Keyboard interface where I decremented the Accumulator from FFh to 00h and then the B register from FFh to 00h. This produced a delay of almost 0.3 secs.


	A nice little 1mS delay for the Z80. In order to use this code with the 8080 and 8085 which has no relative jump (the 20h x) the ordinary Jump if not zero code will have to be used (C2h x x)

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	Hopefully this page of programming examples will be of help to those who may have experienced problems when starting out in writing machine code or assembler. If you need help with something specific that is not here, there are at least many good books around on the subject.