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Text File | 1991-09-08 | 11.5 KB | 294 lines

AMIGA CHIP 8 INTERPRETER & DREAM MON V1.1 ========================================= (C) PAUL HAYTER 1990,91 Does anyone remember the RCA COSMAC VIP computer ? How about the DREAM 6800 or ETI 660 ? I'm not surprised if you don't. These were El-Cheapo Make-It-Yourself Hobbyist computers which appeared in the early 1980's. The COSMAC would be more widely known to U.S. Amiga owners. I'm not exactly sure whether you had to construct the COSMAC yourself, but the DREAM and ETI 660 both appeared in Australian Electronics Magazines as construction projects. What all these computers had in common was that they were disgustingly cheap (about $100), used a hex keypad, could produce ultra stingy 64 x 32 PIXEL (The ETI 660 had 64 x 48 OR 64 x 64 with a modification) graphics for display on a TV, had about ONE kilobyte of RAM, and all ran a pseudo high-level language called CHIP 8 (which was developed by RCA for showing off the COSMAC's graphics, I think). Somewhere along the way, my older brother made up a DREAM 6800. What a computer! Along with the construction articles for the DREAM & ETI 660 were heaps of CHIP 8 game listings. Some of the games were only 200 BYTES or so, so it didn't take forever to type them in. And the games were great fun. They weren't slow. And CHIP 8 was pretty much designed for making Classic style TV games anyway. Alas, the DREAM is now resting silently covered in dust. Even though I now have an Amiga, I somehow yearned to play those hopelessly simple games on the DREAM again. So I've made a CHIP 8 interpreter for the Amiga with a monitor interface (pretty much) identical to the monitor in the DREAM 6800. DREAM MON ========= CHIP 8 can be started from Workbench or CLI. If from the CLI then 1> chip8 Otherwise double click on its icon. You start up in the DREAM monitor environment. The screen is all black except for a white bar across the middle. The white bar shows the current 4 digit HEX ADDR followed by the HEX byte contents of that memory location (NB: these do not correspond to actual Amiga addresses). CHIP 8 in its original form only allowed addressing of a 4Kbyte block of RAM. My implementation does the same, so addresses 0000 thru 0FFF are the only valid ones. The DREAM MON has the following commands: RETURN,=,SPACE,+ :Advance the hex address by 1. -,BACKSPACE :Decrement the hex address by 1. M :Once you hit M you can type in a new 4 digit hex address. G :Executes the CHIP 8 program from the current location. L :Loads a CHIP 8 source (?) file from disk. A file requester appears, so that you can can select a chip8 source file (*.c8). S :Saves a compressed file holding the entire 4K CHIP8 workspace. Again a file requester appears allowing you to enter a new file name, or save as the old name. X :Exit back to the CLI. K :Klears memory. Any hex digit :Typing 2 hex digits in succession will enter that byte into the current location and auto advance the current address. NOTE: DREAM MON starts up pointing to address 0200. This is the default for a DREAM 6800. All programs written for the DREAM are usually entered at 0200 onwards. However, programs for the ETI-660 are entered at 0600 onwards. NOTE: CHIP 8 programs are saved by run length encoding the entire 4K workspace. The compressor works best when the areas of memory which are not used remain blank (= 00). So it is wise to use the Klear command before you type a new program in. An important thing to remember is that this monitor is VERY UN-USERFRIENDLY. You do not hit return after you enter a line. Nor can you correct errors in what you type. However, it is pretty much identical to the original DREAM MON. CHIP 8 EMULATOR =============== After you've pressed G , a chip8 program will either end by itself or stay in an infinite loop. If it is in an infinite loop, then you can return to the DREAM MON by pressing the left mouse button. CHIP 8 ====== CHIP 8 is a kind of high level machine code. No one these days can beleive that a machine code (the actual hex codes making up each instruction) could possibly be "high level", but it is! Each instruction in CHIP 8 consists of 4 hex digits (one word). The 1st hex digit determines the command to be executed, with the remaining 3 digits forming the parameters of the command. The designers have tried to make the first hex digit relate to the command. ie. D for the Display command. Most instructions either operate on a variable and an immediate constant or two variables or just one variable. If we liken CHIP8 to a microprocessor, what we have is a CPU with a 4K address space, 16 x 1 byte registers V0 to VF, and one 12 bit memory index pointer called I. You can load immediate with the 6XKK and AMMM instructions. You can Jump with the 1MMM and BMMM instructions. You can have subroutines with the 2MMM and 00EE instructions. All your condition handling is done with the SKF instructions (usually followed by a GOTO instruction. And the 8XY? instructions allow you to do most arithmetic on the Variables V0 - VF. You have a sort of INKEY$ (from BASIC) instruction with the EX9E and EXA1 instructions. The most powerful instruction is the DXYN instruction which allows you to place a software sprite at any X,Y position on the screen. The image is XORed onto the screen, so you just display it twice to erase it. There are also special instructions to handle numeric output (i.e. game scores). The FX33 instruction stores 3 bytes in memory starting at I. The 3 bytes correspond to the 3 decimal digits of VX. eg. 0200 6AFE VA=FE (254 decimal) 0202 A240 I=240 0204 FA33 MI=DEQ,V3 ... This fragment will store 02 in location 240, 05 in location 241, & 04 in location 242. To use this information we use the FX65 instruction which loads V0 thru VX from memory pointed to by I. For example, continuing the above program: 0206 F265 V0:V2=MI {should set V0=02, V1=05, V2=04} ... Now, to display this 3 digit number we use the FX29 instruction. This sets the I pointer to point to a software sprite pattern corresponding to the least most significant hex digit in VX. Continuing the above: 0208 6410 V4=10 020A 6508 V5=08 020C F029 I=DSP,V0 {I should point to the image of a 2} 020E D455 SHOW 5@V4,V5 {show a 2} 0210 7404 V4=V4+04 {bump X coord} 0212 F129 I=DSP,V1 {I should point to the image of a 5} 0214 D455 SHOW 5@V4,V5 {show a 5} 0216 7404 V4=V4+04 {bump X coord} 0218 F229 I=DSP,V2 {I should point to the image of a 4} 021A D455 SHOW 5@V4,V5 {show a 4} 021C F000 STOP While I won't give a full explanation of how to program in CHIP-8, I will give a run down of the commands which my version understands. CODE MNEMONIC DESCRIPTION 1MMM GOTO MMM Jump instruction at location MMM BMMM GOTO MMM + V0 Jump to MMM + V0 2MMM DO MMM Do CHIP8 subroutine at location MMM 00EE RETURN Return from CHIP8 subroutine 3XKK SKF VX = KK Skip next instruction if VX=KK 4XKK SKF VX <> KK Skip if VX NOT = KK 5XY0 SKF VX = VY Skip if VX = VY 9XY0 SKF VX <> VY Skip if VX NOT = VY EX9E SKF VX = KEY Skip if Key Down = VX; Don't wait EXA1 SKF VX <> KEY Skip if Key Down <> VX; Don't wait 6XKK VX = KK Let Variable X = hex value KK CXKK VX = RND.KK Get random byte. AND with KK. 7XKK VX = VX + KK Add (2's complement) KK to VX 8XY0 VX = VY Let VX = VY 8XY1 VX = VX!VY Logical OR VX with VY 8XY2 VX = VX.VY Logical AND VX with VY 8XY3 VX = VX XOR VY Logical exclusive OR VX with VY 8XY4 VX = VX + VY Add VY to VX. If result > FF then VF = 1 8XY5 VX = VX - VY Subtract VY. If VX < VY then VF = 0, else 1 FX07 VX = TIME Get current timer value FX0A VX = KEY Input hex keycode (wait for keypress) FX15 TIME = VX Initialise timer; 01 = 20 millisecs FX18 TONE = VX Bleep for 20 x VX millisecs (NOT YET IMPLEMENTED) AMMM I = MMM Set memory index pointer to MMM FX1E I = I + VX Add VX to memory pointer FX29 I = DSP, VX Set pointer to show VX (Least significant digit) FX33 MI = DEQ, VX Store 3 digit decimal equivalent of VX FX55 MI = V0:VX Store V0 thru VX at I; I = I + X + 1 FX65 V0:VX = MI Load V0 thru VX at I; I = I + X + 1 00E0 ERASE Clear screen 0MMM CALL MMM Call machine code at MMM (NOT IMPLEMENTED) DXYN SHOW N @ VX,VY Display N byte pattern at coord (VX,VY) 0000 NOP Do nothing F000 STOP Return to Monitor EXAMPLE PROGRAM =============== Just type the hex numbers in at the address shown. The following program moves a box diagonally across the screen. ADDR BYTES 0200 6A00 VA=00 0202 6B00 VB=00 0204 A210 I=210 0206 DAB7 -> SHOW 7@VA,VB 0208 DAB7 | SHOW 7@VA,VB 020A 7A02 | VA=VA+02 020C 7B01 | VB=VB+01 020E 1206 -- GOTO 206 0210 FF81 DATA FOR BOX 0212 8181 0214 8181 0216 FF GETTING STARTED =============== Run CHIP8. You should be presented with a blank screen, with a bar halfway down showing the address $0200 and its contents. Use the left and right arrow keys to step you forward/backward thru memory. Try pressing some hex numbers. You'll only see the first one you type, as the memory address auto advances after you've entered a byte value into it. You can step backwards with the left arrow to see what you've entered. Try pressing K. This will clear the memory to all zeros, removing any hex values you may have typed. Lets load a game. Press L. A file requester will pop up. If you are not currently in the Chip8 directory, you will have to click on a few directories until you arrive. Look at all the files ending in .c8 . These are chip8 source files. Note that some end in 2.c8 and others end in 6.c8 .The ones ending in 2.c8 MUST be executed (pressing G) at address 0200. Conversely the ones ending in 6.c8 must be executed at 0600. Try selecting WipeOff2.C8 and clicking OK. Now to run the game, the memory address on the screen must be 0200. If it is not, type the following 5 keys: M 0 2 0 0 To run the game just press G. This is a simple breakout game. Press keys 4 and 6 to move the bat left and right. If at any time you want to return to the DREAM monitor, press the left mouse button. IN THIS DIRECTORY ================= CHIP8 The DREAM monitor and CHIP 8 emulator. CHIP8.s Assembly source for the above. CHIP8.doc This doc file. The following games are from the original DREAM 6800 article. WipeOff2.c8 WipeOff. 4=left,6=right UFOIntercept2.c8 UFO Intercept. 4=shoot left,5=shoot middle 6=shoot right. Kaleidescope6.c8 Kaleidescope. Move with 4,6,8,2 keys. 0 to start TankBattle6.c8 Tank Battle. Move with 4,6,8,2 keys, 5 =fire 15puzzle2.c8 15 Puzzle. Use 4,6,8,2 keys to move tiles into blank spot. ExampleA2.c8 An example program. ExampleB2.c8 Another example. NOTE: most chip8 games use the cursor keys on the number pad for direction control. CREDITS ======= Michael J Bauer for the DREAM 6800 computer (May, June, July, & August 1979 issues of Electronics Australia). Hugh Anderson & Graeme Teesdale for the ETI-660 computer (Mainly the November 1981 issue of Electronics Today International) Whoever designed the RCA COSMAC VIP computer. Jukka Marin of Supervisor Software for his File Requester (AmigaLibDisk #247). DISTRIBUTION ============ CHIP8 IS FREELY DISTRIBUTABLE. YOU CAN COPY IT AS MUCH AS YOU LIKE AS LONG AS THE FOLLOWING IS SATISFIED. 1. ONLY A NOMINAL COPYING FEE IS CHARGED. 2. IT IS NOT USED FOR COMMERCIAL PURPOSES. NOTE: THE DOC FILE MUST BE DISTRIBUTED WITH THE EXECUTABLE. THE SOURCE FILES CAN OPTIONALLY BE DISTRIBUTED WITH THESE TOO. CORRESPONDENCE ============== Please send any bug reports/ or correspondence to myself at the address below. Paul Hayter PO BOX 331 Ballina 2478 Australia.