>Hi, Hello,
> I received SC-ASM the other day, and being not ill acquainted with z80 >assembler and the workings of the ZX Spectrum, I've programmed a few >basic things. You know, like pressing different keys activating >different border colours and little things like that. I therefore notice >that port 0xfe still does a lot of the same old Spectrum stuff. Yes, 254 (dec) is still the BORDER output port, but with a few differences; most importantly that you can set the border colour to be between 8 and 15 by setting bit 5, and that you can turn off the screen (in modes 3 and 4) by setting bit 7. More about that later. > Now I want to do something better! Like a sprite moving around or >something. Cool. Demos are always a good place to start :) >So, I am wondering - what port & by what method would I page >memory in and out? Right, this is going to sound a bit complicated..... you'll get used to it sooner or later :) The Sam's 512K RAM is arranged in 32 pages of 16K each. The ROM comes in two 16K sections, which go in sections A and D. As far as BASIC is concerned, 16384 is the start of RAM page 0. You can page any two consecutive pages into Z80 addresses 0 -> 32767 (Low Memory, sections A and B), and any two consecutive pages into Z80 addresses 32768 -> 65535 (High Memory, sections C and D). To page RAM into high memory, just output the first page number to bits 0->4 of HMPR = 251 dec (keep bits 5 -> 7 zero, they control external memory and an obscure CLUT feature in Mode 3). To page RAM into low memory, output the first page number to bits 0->4 of LMPR = 250 dec. Bit 5 should be HIGH if the RAM page is to replace the ROM0 page in section A. Bit 6 should be LOW if the RAM page is to replace the ROM1 page in section D. (keep bit 7 zero, it controls write-protect in page A) So, for example: OUT 250,33 OUT 251,30 results in: SECTION A: 0 -> 16383 : PAGE 1 SECTION B: 16384 -> 32767 : PAGE 2 SECTION C: 32768 -> 49151 : PAGE 30 SECTION D: 49152 -> 65535 : PAGE 31 >And where does the screen memory start? Where do you want it to? There is another port, VMPR = 252, which controls the screen location and mode. A mode 1 or 2 screen can be located at ANY page; a mode 3 or 4 screen can be located at any EVEN page. Bits 0->4 are the start page of the screen bitmap. Bits 5 and 6 control the mode (ie. subtract 1 from BASIC's mode number, and shift left a few times). Keep bit 7 zero, it controls MIDI out. Normally, the screen starts at page 30. >If I page out >the ROM can I use the lower 32k for the screen and the higher 32k for >program . . . Yes indeed, though I normally do it the other way round to catch mode 1 interrupts. Note that when you call a machine code routine, it will normally initially be paged into section C (but not if the routine is in page 0 or 2!). So my code normally goes something like this: DUMP 1,0 ORG 32768 in a,(lmpr) ld (lmprs),a in a,(hmpr) ld (hmprs),a and 31 ;make sure ROM1 is paged out and we're not changing anything else or 32 ;make sure ROM0 is paged out out (lmpr),a ;the same memory is paged into sections C and A jp lmem ;jump to the copy in section A hmem: lmprs: equ $+1 ld a,00 ; restore the previous value as BASIC expects out (lmpr),a ret ; end ORG $-32768 defs 56-$ interrupt: ei ret lmem: in a,(vmpr) ld (vmprs),a and 31 ; don't fiddle with the MIDI bit or 96 ; a mode 4 screen out (hmpr),a ... ; do some interesting things vmprs: equ $+1 ld a,00 ; restore BASIC's value in case we changed it out (vmpr),a hmprs: equ $+32768 ld a,00 ; put the program back into section C out (hmpr),a jp hmem ; jump there >and amongst other things neatly dodge the contended memory >issues? Or is contended memory a whole different prospect on the SAM? Contended memory is quite a complex issue on the Sam. Basically, ALL instruction times are rounded up to the nearest 4 t-states, (and execution speed is approximately halved during the screen area. To get some of the speed back you can turn the screen off - as I mentioned earlier - but if you need the display on then basically you're stuck with the contention). See Based On An Idea issue 2 for a more detailed explanation... > Also, I picked up on here that the SAM's video memory is sensibly >arranged. Can I take it that it therefore that it is a true linear frame >buffer in the same sense as a VBE2 (or even good old mode 13h) one is? I >take it one pixel is four bytes? There are four video modes; MODE 1 is exactly the same as the Spectrum. Mode 2 is similar, but the bitmap is laid out left to right - top to bottom. The attribute map starts at an offset of 8192 bytes, and is also composed of 8pixel wide, single pixel high units, in the same order. Mode 3 is 512 pixels * 192 lines, where each pixel is 2 bits, laid out left to right - top to bottom. Mode 4 is 256 pixels * 192 lines. each pixel is 4 bits, laid out left to right - top to bottom. Note that the sixteen colours are an indexed palette, which is changed by outputting a new value to the CLUT, 248, high byte is the index to change. Palette values are calculated: bit value 0 1*B 1 1*R 2 1*G 3 .5*all 4 2*B 5 2*R 6 2*G > I'm not really bothered about sound right now, but which port is that >on? Select a register by outputting to 511, and send the data to 255. I'm not going to tell you which register does what, it's far too complicated - look up the SAA1099 specs or buy a copy of the Sam Technical manual, it'll tell you everything you need to know. To be honest, there's not much point doing your own sound, when it is so much easier to let a package like E-Tracker (or better, ProTracker2) to play music for you... Hope that's enough info for you to be getting on with :) Andrew -- | Andrew Collier | email [EMAIL PROTECTED] | Talk sense to a | Part 2 NatSci | http://carou.sel.cam.ac.uk/ | fool and he +----------------+----------------ICQ:38645805-+ calls you foolish | Selwyn College Student Computer Support Team | -- Euripides
