c64-skills

Commodore 64 BASIC V2: programming rules, the three data types (integer, floating-point, string) and their constants/variables/arrays, expressions, arithmetic/relational/logical operators and precedence, string handling, INPUT/GET, program crunching, and the full alphabetical reference of every BASIC keyword (commands, statements, functions, operators) with abbreviations. Use this skill WHEN reading, writing, or debugging C64 BASIC, or asked things like "what does FOR/NEXT do", "how do I DIM an array", "what's the abbreviation for PRINT", "why do I get ?REDIM'D ARRAY", "operator precedence in BASIC", or "is GET blocking". Pairs with c64-petscii (CHR$/screen codes), c64-keyboard (keys & editor), c64-memory-map (POKE/PEEK targets), and c64-assembly (SYS/USR).

Workflow for reverse-engineering existing Commodore 64 code — .prg/.bin files, cartridge dumps, memory snapshots, and raw 6510 listings. Covers the .prg load address, the tokenized BASIC stub and its SYS launcher, classifying every address as RAM / ROM call / hardware register, recognizing KERNAL ($FFxx) and BASIC ROM entry points, spotting I/O accesses to the VIC/SID/CIA by address, and the common idioms (raster waits, IRQ hooks, self-modifying code). Use this skill WHEN asked to "disassemble this C64 program", "what does this .prg do", "explain this 6510 dump", "find the SYS address", "what is it POKEing $D0xx for", or to trace/annotate a binary. Pairs with c64-assembly, c64-kernal, c64-memory-map, c64-vic-ii, c64-sid, c64-cia.

The two Commodore 64 control (game) ports: reading joysticks, paddles, a light pen, and mouse, and how they map to hardware registers (CIA1 $DC00/$DC01, SID POT $D419/$D41A, VIC light-pen $D013/$D014). Use this skill WHEN a user asks "which port is player 1", "how do I read the joystick", "what bits are up/down/ left/right/fire", "how do I read paddles", "why are paddles unreliable in BASIC", "how does the light pen work", or mentions control port 1/2, $DC00, $DC01, fire button, or POTX/POTY. Pairs with c64-cia, c64-sid, and c64-vic-ii.

C64 screen graphics from the programmer's view: the five VIC-II display modes (standard/multicolor text, ECM, standard/multicolor bitmap), custom 8x8 character sets, screen/color/character memory layout, video bank selection, and smooth scrolling. Use this skill WHEN you see "how do I define custom characters", "turn on multicolor mode", "POKE 53272 / 53265 / 53270 / 53281", "$D011 $D016 $D018 bitmap mode", "switch VIC-II video banks", "where is screen RAM / color RAM", "set the bitmap base", or "smooth scroll the screen". Pairs with c64-sprites, c64-vic-ii (raster timing, full chip spec), and c64-memory-map.

The C64 keyboard and screen editor: every special key and what it does — the Commodore (C=) key, CTRL, RUN/STOP, RESTORE, SHIFT / SHIFT-LOCK, CLR/HOME, INST/DEL, cursor (CRSR) keys, function keys f1–f8, RETURN — plus screen-editor behavior (quote mode, insert mode, full-screen line editing, the 10-key buffer). Use this skill WHEN asked "what does the C= key do", "switch to lower case", "what is RUN/STOP + RESTORE", "clear the screen", "what's quote mode", "type a cursor-down into a string", "reset the C64", or about the f-keys. Pairs with c64-petscii and c64-cia (the NMI/RESTORE mechanism).

The complete Commodore 64 memory map and banking: zero page, stack, the BASIC program area and pointers, screen and color RAM, the $D000-$DFFF I/O area, the BASIC/KERNAL/character ROMs, the $0000/$0001 6510 port plus the 82S100 PLA that switches ROM/RAM/I/O/char-ROM, and VIC bank select via CIA2 $DD00. Use this skill WHEN asking where something lives or how to switch banks: "what's at $D000", "bank out the KERNAL", "C64 memory map", "where is screen/color RAM", "read the character ROM", "what do LORAM/HIRAM/CHAREN do", "which VIC bank am I in", "zero-page usage". Pairs with c64-assembly, c64-vic-ii, c64-sid, c64-cia, and c64-kernal.

Commodore 64 hardware sprites (MOBs, movable object blocks): the 24x21-pixel / 63-byte sprite shape, sprite pointers, enabling sprites, X/Y positioning including the X MSB ($D010) for positions past 255, sprite color, multicolor sprites, 2x X/Y expansion, sprite-to-background and sprite-to-sprite priority, and collision detection. Use this skill WHEN you see questions like "how do I make a sprite on the C64", "POKE 53269 enable sprite", "move a sprite past x 255 / $D010 MSB", "$D000 $D015 $D017 $D01C $D01D sprite registers", "set sprite pointer 2040", "multicolor sprite", "expand a sprite", "detect sprite collision $D01E $D01F", or "sprite priority". Pairs with c64-graphics (screen/charset/bank setup) and c64-vic-ii (exact raster timing and chip behavior).

The physical Commodore 64 and its servicing: system specs, the chip complement (6510, VIC-II, SID, two CIAs, PLA, ROMs, DRAM) and each chip's role, the circuit theory (power, reset, clock, video/audio, ports), board revisions and how to ID them, connector pinouts, and troubleshooting. Use this skill WHEN a question is about C64 hardware, a board, or repair — "what chip is U17", "why won't my C64 boot", "blank screen on power-up", "power-brick voltage", "which board revision", "5-pin vs 8-pin video", "expansion port pinout", "PLA failure". Pairs with c64-memory-map, c64-vic-ii / c64-sid / c64-cia, and c64-io / c64-game-ports.

The three C64 character-code systems and putting text on screen: PETSCII (what PRINT and CHR$/ASC use), screen/display codes (a DIFFERENT mapping you POKE into screen RAM at $0400), and ASCII; plus cursor/color/reverse control codes and building text UIs with PETSCII box/line graphics. Use this skill WHEN asked "what CHR$ code clears the screen", "why does POKE 1024,1 show an A", "PETSCII vs screen codes", "how do I print in red", "draw a box/menu in PETSCII", "color code for cyan", or "POKE to screen RAM". Pairs with c64-keyboard, c64-graphics, and c64-memory-map.

Cycle-level hardware reference for the C64 video chip (MOS 6566/6567/6569 VIC-II): the register map ($D000-$D02E), 16-color palette, raster mechanics ($D012 + $D011 RST8), Bad Lines, the memory-access timing model, all eight ECM/BMM/MCM graphics modes (incl. the invalid ones), sprite DMA/priority/collision, borders, light pen, interrupts ($D019/$D01A), DRAM refresh, PAL vs NTSC timing (6569 vs 6567), and raster effects (FLD/FLI). Use this skill WHEN you need emulator-accurate timing or register/bit detail: "cycles per raster line on PAL", "what is a Bad Line", "set a raster interrupt", "the 9th raster bit", "open the border". For drawing see c64-graphics / c64-sprites; $DD00 VIC bank bits see c64-memory-map; CIA IRQ wiring see c64-cia.

Writing 6510 machine language on the Commodore 64: the CPU (a 6502 plus the on-chip $0000/$0001 I/O port that banks ROM/RAM), registers and status flags, the 13 addressing modes, the full 56-instruction opcode set, hexadecimal, the 64MON monitor, where to place ML routines, and calling ML from BASIC. Use this skill WHEN writing or reasoning about C64 assembly: "write a 6510 routine", "what does LDA #$01 do", "how do I SYS to my code", "pass A/X/Y from BASIC", "where do I put machine code", "what's the USR vector", "RTS back to BASIC". Pairs with c64-kernal, c64-memory-map, and c64-disassembly.

The two MOS 6526 CIA chips on the Commodore 64 — CIA1 at $DC00 (keyboard scan, joystick/paddle ports, the IRQ source) and CIA2 at $DD00 (serial IEC bus, user port, VIC bank select, the NMI source) — their data ports A/B and DDRs, the two 16-bit interval timers, the Time-of-Day clock, the serial shift register, and the interrupt control register. Use this skill WHEN working with these chips: "set up a CIA timer IRQ", "read the TOD clock", "what's at $DC0D", "the CIA interrupt control register", "$DD00 VIC bank bits", "user port lines". Pairs with c64-keyboard, c64-game-ports, c64-io, c64-vic-ii, and c64-memory-map.

Using a Commodore disk drive (1541 and family) from the Commodore 64: device 8, LOAD/SAVE, listing the directory, the command channel (channel 15) for DOS commands (NEW/format, SCRATCH, RENAME, COPY, INITIALIZE, VALIDATE), reading the error/status channel, and file types (PRG/SEQ/REL/USR). Use this skill WHEN a user asks "how do I show a disk directory", "LOAD a program from disk", "what's the difference between LOAD\"NAME\",8 and ,8,1", "how do I format/scratch/ rename a disk file", "what is OPEN 15,8,15", "how do I read the disk error channel", or mentions device 8, the 1541, or a .d64. Pairs with c64-io, c64-tape, and c64-kernal.

The Commodore 64 input/output system and its device/file model: logical device numbers, OPEN/CLOSE/PRINT#/INPUT#/GET#/CMD, secondary addresses, and the ports behind them — output to TV/printer/modem, the RS-232 interface, the serial IEC bus, the user port, the expansion (cartridge) port, and the Z-80 cartridge. Use this skill WHEN a user asks "how do I OPEN a channel", "what device number is the printer/screen/RS-232", "what does the secondary address mean", "how does the serial bus work", "what's on the user port or expansion port", "how do I set up RS-232 / a modem", or works with EXROM/GAME, ATN/CLK/ DATA, $DD00, $DE00. Pairs with c64-disk, c64-tape, c64-game-ports, c64-cia, c64-kernal, and c64-memory-map.

The Commodore 64 KERNAL ROM jump table and its user-callable routines: the fixed $FFxx entry points, each routine's purpose and register inputs/outputs, the carry-flag error convention and error codes, and the power-up sequence. Use this skill WHEN calling C64 OS routines from assembly: "print a character in 6510", "address of CHROUT/CHRIN/GETIN", "open a file with SETLFS and SETNAM", "the KERNAL I/O pattern", "what does set carry mean after a KERNAL call", "read the jiffy clock with RDTIM", "scan the STOP key". Pairs with c64-assembly, c64-memory-map, c64-io, c64-disk, and c64-tape.

The MOS 6581 SID sound chip and making sound and music on the Commodore 64 — the register map at $D400-$D41C, the three voices, the four waveforms (triangle, sawtooth, pulse, noise), the ADSR envelope generator, pulse width, the multimode filter, master volume, ring modulation and oscillator sync, and the frequency/note tables. Use this skill WHEN asked to "play a note/sound on the C64", "program the SID", "what does $D418 do", "set up an ADSR envelope", "make a sound effect", "why is my SID silent", or any POKE/STA into $D4xx. Pairs with c64-game-ports (SID reads the paddles), c64-memory-map, c64-vic-ii and c64-cia (raster/timer-driven music players).

Orientation and router for Commodore 64 programming and hardware — the architecture at a glance, the memory/IO map, and a "which skill answers this" index across the focused c64-* skills (BASIC, graphics, sprites, VIC-II, SID, 6510 assembly, KERNAL, memory map, CIA, I/O, disk, tape, game ports, PETSCII, keyboard, hardware, disassembly). Use this skill WHEN a C64 question is broad, vague, or cross-cutting ("how does the C64 work", "where do I start", "what chip does sound"), when you need the high-level memory map or chip locations, or to route a specific question to the right c64-* skill. For depth, defer to the specific sibling skill.

The Commodore 64 Datassette / cassette tape: device 1, LOAD/SAVE/VERIFY, how tape files are found (sequential, no random-access directory), the tape buffer, headers, and the PRESS PLAY/RECORD prompts. Use this skill WHEN a user asks "how do I load/save from tape", "LOAD with no name", "how do I list a tape directory", "what's on this cassette", "why does it say PRESS PLAY ON TAPE", "what is the tape buffer at $033C", "how do I write data files to tape", or mentions the Datassette, device 1, or .tap/.t64 files. Pairs with c64-io, c64-kernal, and c64-assembly.