Your Spectrum 07 - Channels & Streams

Your Spectrum
Issue 7, September 1984 - Channels & Streams

**See also the first half of this article:**
Issue 5	"Spectrolysis: Channels & Streams"

SPECTROLYSIS

As promised in our July issue, Ian Beardsmore continues his quest to open up a new stream to a new channel. Read on and check out his progress ...

	ASSEMBLER	COMMENTS
This is a short set-up routine, making some room in memory for our new channel.	LD HL,(PROG)	Because the precise place we want within the channels area can be awkward to find, here is a little bit of cheating as we can go backward from PROG.
	DEC HL	This is a prerequisite of the CALL we are going to use.
	LD BC,0005	Another prerequisite. This is the size of the space we want to make.
	CALL #1655	This is a call to the 'make space' routine within the Spectrum's ROM. HL has to point to the first byte after the space and BC needs to hold the length of the space. When the program has RETurned from the 'make space' routine, HL will hold the last byte of the space.
We can now fill our newly created space in the CHANS area with our own data. A check back to last month will reveal that the first two bytes will form the output routine (the one we will use), the next two form the input routine and lastly, the specifier (which must be K, S, R or P). We will stick to Channel 3 and use 'P'.	INC HL	HL now holds the address of the first byte of our extra space.
	LD A,#94	The lowest significant byte (LSB) of our output routine.
	LD (HL),A	This is now loaded into the first of our free addresses.
	INC HL	The next of our extra addresses go into HL.
	PUSH HL	We are going to need this address again later.
	LD A,#F2	The most significant byte (MSB) of our routine.
	LD (HL),A	This is loaded into its allocated address.
	INC HL	The next free address - the start of the input routine.
	LD A,#C4	The LSB of the input error routine.
	LD (HL),A INC HL	Which is stored away. The next free address.
	LD A,#15	And the MSB for it.
	LD (HL),A	Is stored away.
	INC HL	The next address.
	LD A,#50	Is the code for the letter 'P', our specifier.
	LD (HL),A	Without a recognised specifier, we would go to an error routine whether we wanted to or not.
Having made the space of our channel and loaded it with the data we want, the offset has to be found so that it can be stored in the STRMS area.	POP HL	The first address plus one of the extra space that we stored with a PUSH earlier.
	LD DE,(CHANS)	The vector that will return the start address of the channels area to DE.
	AND A	Clears the carry flag ready for the calculation.
	SBC HL,DE	The difference between the start of the channels area and the start of the extra space we've made becomes our offset; it's currently held in HL.
	EX DE,HL	We need to keep the offset, but HL is in demand for other things. A simple swap, then, is the answer. HL is free for other uses, and the offset is in DE. All that's needed now is to find the place in STRMS where the channel offset can be stored.
	LD HL,'STRMS'	This is not a vector, and has to be loaded as an absolute value or else HL will be incremented.
	LD A,4	The stream we are interested in goes into the A register.
	ADD A,3	This allows for Streams -3 to -1.
	ADD A,A	Each of the seven streams that come into use on powering up the Speccy have two bytes of offset data allocated to them. The total number of bytes they occupy is now in the accumulator and forms our streams offset for Stream 4.
	LD B,0	It must not be forgotten that, even though we're using low value offsets, an offset can still be a two-byte number and in these cases must be treated as such.
	LD C,A	The LSB of the offset.
	ADD HL,BC	The streams offset is added to the streams base address, to give the location we want in the streams area.
	LD (HL),E	The LSB of our channel offset goes into the lower of the streams addresses.
	INC HL	Points to the next streams address.
	LD (HL),D	Loads that address with the MSB of our channel offset.
	RET	RETurns to Basic.

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Last issue, I was rambling on about streams and channels, hoping to excite the nerve endings of any readers out there who could help me in my quest to open up this little-known area of Spectrum knowledge. Well, for all those of you still asleep in the back row, I've managed to come up with most of the answers - so, this month, I will be providing you with a routine to open a new stream to a new channel!

OPENING TIME

Opening a stream can be performed simply though various POKEs - the problem comes in creating a new channel. Whereas the streams area is absolute (the start of which is pointed to by the systems variable STRMS) the systems variable CHANS holds an address or vector which can, and does, move.
All this work was done on a Spectrum devoid of an Interface 1 unit, though the routine should be usable with said unit attached. Unexpanded, the Spectrum uses seven channels: -3 to -1 are for internal use, leaving Channels 0 to 3 for us to do something with. However, if we do use one of these it will have to be at the expense of the screen, keyboard or printer (yer pays yer money, etc). Much better, then, to create new space filled with channel data of our own channel - hence the need to CALL the 'make space' routine from the ROM!
As far as Sinclair Research is concerned the stream number is synonymous with the channel number, and as such is a source of confusion. While it's a useful rule of thumb, it's not a necessity. In theory, it would be quite possible to have, say, 16K of channel space ... but there are limitations - namely that only stream and channel numbers between zero and 15 (inclusive) will be accepted. In the example I have given, the output routine of our new channel - Channel 4 - points to an

	SPECTROLYSIS
address 62100, where it is presumed that a routine of our own devising would be stored. A note about offsets is also needed. An offset is a number that, when added to the base address of a block of data, will give you the address to start working from. The routine included in this article uses two offsets: one a channel offset, to show where the channel data for our routine starts; the other, a streams offset, is the number of bytes into the streams area that the above offset is stored. I said last month I was hoping to assemble a routine that would allow you to attach your own streams to your own channels - the channels being your own routines. This proved even more awkward than I had expected and took some time to accomplish. For the present, I'll only give the assembly listing and explanation. One point to keep in mind when trying to CLOSE channels greater than Channel 3 - there's no endstop marker on the Lookup Table, which might just have some of you whizzing merrily through the ROM until the program finds some number that's acceptable.	CRASH THEORY CONTINUED Further investigations into the 'Fun Crash' I gave you last month (well, it's a lot less energetic than a Fun Run!) have shown that CLEAR 64000 is not quite unique, even though it does seem to have some properties that are distinctly its own. But, in fact, a similar effect can be obtained with these addresses - 33024, 41472 and 51465; can anyone spot what they have in common? If highly intelligent readers of Your Spectrum cannot, then no one will. And what if I add a couple more addresses that can also be used, such as: 32000, 32256, 32512, 49152 ...? Yes, no prizes for knowing that any address with a low byte value of 00 Hex will produce a similar sort of crash, though with nothing like the same spectacular pattern. I started to wonder what was so special about every 256th number that it produces a screen display,	whereas any other (leastways, the others I've tried) results in a simple crash. Then I discovered that the incomparable Sir Clive had another trick up his sleeve - my fine theory itself crashes, literally, at address 52736. So far as I know, only one address above this produces a display before the crash - when the computer NEWs itself - and that's the well-tried (and far more interesting) 64000. But, hang on ... "Before the computer crashes and NEWs itself ...", do I hear you exclaim? That wasn't mentioned before! Well, I was just testing your alertness. The fact is that though 64000 seems to produce a continuous display, all the other addresses mentioned earlier do it to a regular time sequence. Here's a very exciting graph - well, maybe not that exciting, but still quite interesting. The y axis represents time in minutes, and the x axis the address (always remembering that only every 256th address applies). There's a clear relationship between when the display starts coming up on-screen and when it NEWs itself. Obviously, the three addresses - 52480, 52736 and 52992- are the interesting ones, as here the pattern is completely broken, never to be repeated. I have left the 64000 display on for half an hour without it NEWing, so I make the assumption that it's permanent. Now, all I want are some explanations ... any takers?

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OPENING TIME

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