The Incredible KIMplement

• Powered by "6o6" -- a full software NMOS 6502 emulation (documented opcodes only, sorry) with all addressing modes, simulated ROM, and simulated memory-mapped i/o
• LED and keypad emulation via partial RIOT emulation
• TTY emulation (ASR-33 subset) via partial RIOT emulation
• SST emulation
• 4K RAM expander card
• KIM-4 emulation with 8K RAM expander card (thus total address space 16K)
• Revision E ROMs built-in, so you don't have to find a pirated set download them from your own KIM
• Load and save RAM dumps to disk

In the future, more hardware support will be added to the KIMplement to make it as comprehensive an emulator of the KIM-1 as possible within the constraints of the host system.

Only a stock Commodore 64 or compatible emulator is required. Simply LOAD and RUN the program to start. After the title screen, the emulated KIM will auto-reset, the LEDs will appear, and the time machine will transport you to the year 1975!

If you're an impatient little so-and-so, you can jump to the goodies, but I recommend reading the below first, especially if you have never used a KIM-1 before.

Using the KIMplement

The KIM-1 has six seven-segment LEDs, displayed at the top of the screen as the rightmost six LEDs of seven (the seventh LED on the far left is the KIMplement mode indicator). In the rudimentary KIM monitor, the address is displayed on the four left digits in hexadecimal, and its current contents on the rightmost two.

The KIM-1 also has a small hexadecimal keypad along with several special keys: AD to set the current address, DA to enter data, + to advance one byte, PC to see the last program counter, GO to start execution from the current address, ST to trigger an NMI (stop execution), and RS to send a reset to the machine (the contents of memory are not altered).

On startup, KIMplement displays a 'P' for 'keyPad' in the mode indicator (leftmost LED) indicating that the keyboard sends the emulated KIM presses on the keypad. Keys 0-9 and A-F send the appropriate hex digit. + sends the plus sign key. R (for 'addRess') sends an AD keypress, G sends a GO keypress, and T (for 'daTa') sends a DA keypress. The keys to send ST and RS are special and will be discussed presently.

KIMplement can also emulate a TTY connected to the emulated KIM-1 (there's a screenshot of this to the right); this emulation is modeled on an old-school ASR-33 teletype. The TTY emulation mode is triggered by pressing RUBOUT, which is mapped to the INST/DEL key. Press the INST/DEL key firmly and hold it briefly, as there is a race condition in the KIM-1 ROM accentuated by the slower system speed. The mode indicator will change to 'b' for 'keyBoard' indicating TTY mode. In this mode, KIM types out the current address and contents to the console and waits for input. There is no punch tape reader function, although you can try to save to punched tape (the data stream is simply echoed to the console). Use the same keys indicated in the KIM-1 User Manual for sending TTY commands. Special ASR-33 keys and control sequences are mostly supported; here are the ones in this version:

• RUBOUT ($7f, $ff): INST/DEL
• BACKSPACE ($08): CRSR LEFT or CRSR RIGHT (this isn't an ASR-33 key but is provided for convenience)
• XON ($11): CRSR DOWN
• XOFF ($13): HOME
• LINEFEED ($0a): F1

If the TTY isn't responding to commands, the race condition probably got triggered. Press INST/DEL, repeatedly if needed, until the TTY responds. Remember to press it firmly. This symptom is a lot less frequent now than in previous versions because the CPU emulation is faster, but still possible to trigger.

You may freely switch between keyPad and keyBoard modes at any time (many programs will only take input from the keypad, for example). To silently enter keyBoard mode from keyPad without sending RUBOUT, press SHIFT (the screen will turn white) and release it. Note that since SHIFT is obviously an active TTY key, to shift back to keyPad mode, hold down SHIFT and press the Commodore key (the screen will turn cyan), and then release both keys. The mode indicator will always show what type of input method KIMplement is facilitating, but note that this will not change where the KIM monitor or the currently running program is expecting input from -- if you were in keyPad mode, you must return to keyPad mode to get the monitor to respond to your commands, for example. Similarly, programs hardcoded to accept information from the emulated TTY must be in keyBoard mode, such as TinyBASIC. Remember that to get the KIM monitor to acknowledge the TTY, you must press rubout (INST/DEL) to switch the KIM-1 to TTY mode as described above, and to once again get the KIM monitor to accept keypad input, the emulated KIM must be reset (see the entry on the RS key below).

To stop the currently executing program on a KIM-1, an NMI (Non-Maskable Interrupt) is sent by pressing the ST key on the keypad. In keyPad mode, the CTRL key is mapped to ST; press CTRL (the screen will turn purple) and release it. Since CTRL is also an active key in TTY mode, hold down CTRL and press the Commodore key (the screen will turn purple), and then release both keys, to trigger NMI in keyBoard mode. The emulator will automatically switch to keyPad mode on an NMI. For the ST key to properly operate on a KIM, as well as in the KIMplement, the NMI vector at locations $17fa-b must have bytes $00 and $1c in them respectively. To enter this in keyPad mode, use the following sequence:

[AD] 1 7 F A [DA] 0 0 [+] 1 C [+]

In keyBoard mode, use the following sequence:

1 7 F A [space] 0 0 . 1 C .

If this vector is not set, the emulated KIM-1 may crash or lock up in an endless loop on NMI! If that happens, you will need to reset the KIM with the Commodore key; see below.

During operation, the emulated CPU may return error or information conditions to the KIM emulator. When a CPU condition occurs, the border will flash and the emulator will report the condition and the current emulated 6502 state. BRK is handled "transparently" since many KIM applications use it as a software interrupt; it will flash the border white-and-black, then automatically trigger the IRQ, load up the stack, set the emulated B-flag and continue execution through the 6502's IRQ vector as usual. However, nothing is printed on the screen otherwise to avoid interference with normal operations. The other messages are fatal errors and the emulator will force you to non-destructively reset the emulated CPU (illegal instruction, memory fault, consistency error [bug in the CPU emulation! ack!], etc.). This will restart the KIM, but memory should remain intact (assuming the errant program hasn't trashed it). Do note that for BRK instructions to properly function, the vector at $17fe should be properly set (or set to $1c00, as in the example above for $17fa). If you get an endless loop of BRKs, you will need to reset the machine (see below).

The main menu, along with the reset command, is accessed with the Commodore key in both keyPad and keyBoard modes. Press the Commodore key (the screen will turn red) and release it; the emulator will pause, display the current state of the emulated CPU, and present options. From here, you can:

• press the Commodore key again to trigger a non-destructive CPU reset (the same as pressing RS on the keypad), which will terminate the current running program but will not disturb memory -- this will set the emulator back to keyPad mode;
• press 8, 9, 0 or 1 to set the default disk drive device number to 8, 9, 10 or 11 respectively;
• press @ for the built-in DOS wedge (allows you to send disk commands, list the disk directory or read the error channel);
• press L or S to load or save data from/to disk respectively;
• press T to access the toggle switch menu; or
• press R to resume emulation.

The SST mode is important and deserves additional explanation. When SST mode is activated, for any code that is not in ROM (at locations $1800-$1fff), an NMI is triggered immediately after each instruction executes, thus allowing single stepping through code (press GO or type 'g' depending on keyPad/keyBoard mode to keep stepping). Because SST mode uses NMIs to single-step, the NMI vector must be set at $17fa-b as described above, or the emulated CPU may crash! The emulator automatically switches back to keyPad mode on each step.

To load or save files, type 'L' or 'S' respectively from the main menu. To load a file from disk, the file should be stored as a regular .prg file on the default disk drive. KIMplement prefers that such files have (and files saved will have) an apparent starting address of 16384 ($4000) plus the starting address in the emulated KIM's addressing space; this is because the KIM-1's RAM is stored at this location in the C64, with the addressing space extending in the C64 from $4000-$7fff. However, since there are many raw binaries without a starting address, KIMplement will also allow you to specify a starting address for those types of files. To indicate that there is a valid starting address on the file, after pressing L, press K to indicate that this is a KIMplement-style .prg file. Otherwise, press B to indicate it is a raw binary and the KIMplement will ask you where you want to load it. Note that KIMplement rigidly enforces its addressing range and will refuse to load a program that doesn't have a valid starting address in this 16K block, and if the loaded file extends outside of this range KIMplement will force you to cold start the emulator from scratch to protect itself, potentially losing your work! To cancel a load, simply enter a blank filename. On a successful load, KIMplement will report the starting and ending addresses (inclusive) of any memory locations changed as a verification measure.

You will note that since the ROMs lie between $1800 and $1fff (real locations $5800 and $5fff), you can overwrite them with your own ROMs. This should be considered a feature, and is useful and even supported, but there are important emulator-specific limitations you should read first.

To save a file from disk, enter the desired filename, and then the starting and ending addresses (inclusive; do not enter ending address + 1) in hexadecimal. Give the addresses relative to KIM RAM; KIMplement will save the file to disk with the proper starting address and return the DOS error code, if any. Enter a blank entry at any time to cancel the save operation.

Technical and compatibility notes

KIMplement uses a KIM-4-style memory map for its expanded RAM emulation, with two changes: first, RAM exists in *every* location between $0000 and $1700, whereas a real KIM with most 4K low-RAM expansion boards typically only had RAM up to $1400. This is done for efficiency reasons rather than special-casing this little-used section of addressing space, but if you want your programs to be guaranteed to work on as wide a range of configurations as possible, don't take that block of RAM between $1400 and $1700 for granted. Second, above $4000, the unmapped addressing range is hardwired to return $ff on purpose; this forces an immediate illegal instruction exception if the program counter gets into this range, greatly facilitating debugging. KIMplement correctly mirrors $17f8-$17ff to $fff8-$ffff, just like a real KIM-4.

There are certain elements of the KIM hardware that KIMplement does not presently emulate. In particular, the RIOT emulation is incomplete; besides only kludgy support for terminal and keypad lines, the most important deficiency is interval timer support. A full-speed and completely cycle-accurate software emulation will never be possible on the stock ~1MHz Commodore 64 because the timers also offer a mode where every single pulse of the KIM's own 1MHz clock is counted by the timer -- impossible because the instructions merely to do the counting, let alone the 64's own IRQs and housekeeping and the rest of the emulator itself, require much more time than a single clock cycle. I am considering adding a feature where the right number of ticks are counted, but in groups based on the 60Hz Timer A IRQ, or possibly even using the 64's CIAs to count 64's own clock pulses at its similar clock speed as long as this does not greatly impact performance -- since the sorts of applications needing this support typically use it for real time triggering, it's important that the interval timers themselves run in real time, not by counting slower "emulated clock ticks." However, I haven't implemented either scheme yet and in any case either option might still not be accurate enough for certain applications. (The rest of the RIOT feature set is not so constrained and I will gradually add more accurate I/O sensing and similar features into future versions.)

A related problem is that several segments of the KIM-1 ROM were written assuming that the CPU ran at a fixed 1.000MHz -- a real trick when the 64's own 6510 is itself running at approximately that same speed as we explained above. There is no way the stock C64 can keep up with this, so for these problematic areas of ROM, the emulator actually traps calls to these routines and replaces them with native kernel-level code. In particular, the routines to flash the LEDs, read the keypad, send characters to the TTY and receive characters from the TTY have been replaced with native-mode traps, and certain startup routines are also diverted.

Because this requires the emulator to trap the emulated 6502 at certain locations in ROM, this also means that these locations must be valid for arbitrary entry. Currently, the following routines (referenced by symbol name in the KIM-1 disassembly as it appears in the User Manual) must remain at their documented entry points:

• DETCPS ($1c2a)
• START ($1c4f)
• GOEXEC ($1dc8)
• GETCH ($1e5a)
• OUTCH ($1ea0)
• AK ($1efe)
• SCANDS ($1f1f)
• KEYIN ($1f40)
• CONVD1 ($1f5b)
• GETKEY ($1f6a)

As previously mentioned, you can replace the built-in ROMs with your own ROMs, but because of the emulator's current architecture your custom ROMs must obey all of the entry point constraints noted above (both to enable compatibility with KIM-1 application programs, and to make sure that ROM traps do not fail or the emulator may trap routines falsely or (worse) place the PC at the wrong instruction). Be careful with the length of your code as well: if the length of your ROM file(s) cause them to expand into KIM RAM above $2000 (real C64 location $6000), then they may not properly be write-protected because the emulator does not know they are supposed to be "ROM," and if you really go overboard and cause them to expand beyond $3fff (real C64 location $7fff), then you will corrupt the emulator, be forced to coldstart, and lose your work! To replace 6530-002, your code should be 1K in length and have a starting address of $5800 (assembled with * = $1800); to replace 6530-003 (the master ROM), your code should be 1K at length, must have valid NMI, IRQ and RESET vectors in the last six bytes!, and have a starting address of $5c00 (assembled with * = $1c00).

Unfortunately, the emulator cannot do this sort of trapping for arbitrary programs in general. Programs that depend on the interval timer may not work at all (although some programs used the LSB at $1704 as a cheap source of 'random' numbers, and in this version this shameless practise is supported [weakly] by simply copying the LSB of the C64's jiffy clock there on each emulated instruction execution). Programs that use a delay loop based on a certain number of processor cycles will run on the KIMplement, but the delay loop will be considerably longer in real time. (This is the reverse of compatibility problems with the CMD SCPU, where a program that assumed the speed of the 64 was fixed ran too fast instead.) For such programs, the delay loop will have to be recalibrated, or the program rewritten to wait for a keypress or some other external signal. Eventually these routines might be rewritten for the interval timers when this support is added.

One other note is that SCANDS, the KIM-1 ROM routine that flashes the KIM data/address LEDs and refreshes them, depends on the LEDs having some kind of afterimage to make a smooth display which the sprites simulating them obviously don't have. The ROM SCANDS operates by turning all the LEDs off after briefly lighting the display, which when in a loop at the speed this emulator runs at, causes noticible flickering. Instead, the emulated SCANDS trap (enabled by default) leaves the LEDs on after setting up the display for a smoother-looking appearance, extinguishing them only when direct access is made to the LEDs, or when a program is started. Usually this works, but if the program depends on the LEDs being extinguished immediately afterwards, the display may not function correctly. For these programs, go into the toggle menu and enable ROM SCANDS to use the old version, but have your headache medicine handy when you see what happens to the LEDs.

To-do list, and known bugs

• Not a bug in the emulator per se, though easier to trigger in it: a race condition exists in the KIM-1 ROMs where if the rubout key is released too quickly, the ROM code detects the key, but the bits in SAD are no longer set when the ROM checks for rubout again in the START routine since the key is no longer down. The TTY will then appear to be 'stuck' (actually, the ROM monitor is just in a loop). This bug is easier to trip in the KIMplement where the effective CPU speed is much slower, although as of 0.2b it should be much rarer. Neverthelesss, if the TTY seems locked up, just press INST/DEL again firmly, and repeatedly if necessary, until the TTY responds normally.
• Interval timer support is not in this version (see above).
• Keypad and terminal lines on SAD are only partially emulated (see above). To read keys from either source, use the documented ROM routines for now and avoid strobing the lines yourself.
• There is no tape support. Although the file management menu mitigates this, some programs (MicroAde, for example) use the tape drive as intermediate storage and some others even use it to generate music. This last is a little exotic (see Jim Butterfield's "Music Box" in "First Book of KIM"), but the first should be doable on a disk drive with a little creativity. It will undoubtedly require some additional traps to be installed. I will come up with a scheme for 0.3.
• There is no punch-tape reader. Reading paper tape seems superfluous since the load and save functions also cover this, but sometimes that's the only format available. To solve this problem for now, look at paptap.pl below in the download section, which will read in punched tape that has been turned into ASCII text and turn it into binary files (for example, this is Tiny BASIC). This is a Perl script that should run on any implementation of Perl. Eventually punchtape support will be incorporated into KIMplement if there are enough requests for it.
• Supporting one of the video boards, which should be a slam-dunk on the C64. I will probably write a patch to allow the $2000-$3fff range to be treated like a Visable board, although this will require some remapping for the 64's wonky hi-res scheme. Likely I will achieve this by only doing hi-res writes when the screen is enabled, and doing a "catchup" routine when the screen is toggled on to do all the remapping en masse so that text-mode processing is not unnecessarily impacted. The $e000-$ffff area in the 64 seems the best place to mirror the converted hi-res display.

Okay, now the goodies.

If you're lazy and don't want to download and run these .sdas or can't be bothered to figure out Star Commander, or just want to emulate the emulator because you're uncool, you can download all of these together in a .gzipped .d64 which can be turned into a real 1541 disk using Pasi Ojala's gunzip.c64. However, please, please, PLEASE read the instructions below anyway. kim2d64.d64.gz (20K)

• Current version 0.2b (11K). This is a .prg file runnable directly on the C64 or on any modern Commodore 64 emulator. It is freeware, offered under the Floodgap Free Software License.

  This program is pre-crunched with pucrunch. It takes a few seconds to decompress and depack itself; on the 128, it will kick the 8502 to 2MHz mode temporarily to speed this up, so the screen will get garbled for a second or two. This is normal.

  Here's the changelog:

  • 0.2b:
    • Emulation speed about 2-5% faster due to additional 6o6 optimizations and further streamlining of the main loop.
    • Better TTY handling. In particular, TTY keys don't progressively get out of sync when the emulated KIM is working, and RIOT bits are more accurately modeled. It is still possible to drop keystrokes by overflowing the Kernal keyboard buffer but this should happen less frequently in normal use.
    • Patched out a race condition where the emulated KIM could set the decimal flag right at the time the C64 would execute a native interrupt or NMI, causing a crash (the Kernal doesn't account for this ordinarily).
    • Works around a VICE bug where testing for the existence of a file in a host machine directory with RENAME gives error 0, not 63. This allows you to load files from a host directory in the emulator; previously it only worked from .d64s.
    • KEYIN trap added.
  • 0.2a:
    • Fixed error in KIM-4 memory addressing.
    • Corrected GETCH trap.
  • 0.2:
    • Smaller, pre-crunched executable (about half the size).
    • Overall emulation speed is about 25% faster, thanks largely to improvements in the CPU core, and some optimization in the memory handlers and emulator main loop.
    • KIM addressing space expanded to 16K ($0000-$3fff), using a KIM-4-like scheme.
    • TTY emulation improved: SHIFT and CTRL properly monitored in TTY mode, ASR-33 character set fully supported, BEL added, LF moved to F1 to allow backarrow, XON/XOFF on CLR HOME and CRSR DN, BS on CRSR LF (and RT for single key typing); smarter handling of LF and CR LF sequences.
    • RIOT emulation improved: correct bits set for TTY activity.
    • BRK made "transparent" but still has brief visual notification for debugging.
    • Disk wedge and multiple device number support added.
    • Loading binaries without Commodore-style starting addresses supported.
    • Saving RAM to disk works properly now.
    • Backspace-on-resume bug fixed.
    • Stuck keys getting echoed into the menu fixed.
    • Le funky musique.

• Demonstration programs (3K). This self-dissolving archive, when run with a disk in device 8, will dissolve down to five applications consisting of ten individual files (Microchess is multipartite; the others are single-file). All programs are the copyrighted property of their original authors and appear here by permission of the original copyright holders. We appreciate their generosity. Please respect the author(s)' rights in your usage of these programs.

  By the way, these programs have not been modified in any way other than to add a starting address, and will work on a real KIM (with the starting address removed) too. You should load them as "K"IMplement format, not as raw binaries.

  • Addition by Jim Butterfield, reference "First Book of KIM" page 24 (Hayden). Starts at $0200. Simple decimal adder using number keys and plus sign; GO to clear total.
  • Key Train by Jim Butterfield, reference ibid., page 72. Starts at $0000. Press the key displayed. Mostly useful as a self-test. :-)
  • Black Match by Ron Kushnier and editors, reference ibid., page 46. Starts at $0200. The rightmost two digits display how many matches are left to pick; the goal is to make the computer pick the last match. The count starts at 21 (get it? "black match"?), similar to the Nim variant misère game 21 but played in reverse: you can only take 1, 2 or 3 matches per turn (press that key). The computer's move is shown in the middle two digits. KIM's game adapts to your skill level -- keep winning and it'll start sharpening its knives. When you see "SAFE" (you win) or "DEAD" (you didn't), press GO for a new game.
  • Lunar Lander by Jim Butterfield, reference ibid., page 84. Starts at $0200. Use 1-9 to control your engine throttle (left four LEDs are altitude/fuel [press 'A'ltitude or 'F'uel to determine which], and right two is descent). Try to land with a descent speed of 5 or less ("SAFE"), or else KIM displays DEAD. Pressing 0 kills your engine. GO starts a new game. If the game takes too long to play due to the lower CPU speed, or you want to try BASE jumping on the moon, you can start at a lower altitude by changing byte $02CC to a lower number. This figure is in BCD, so the default $45 starts you at 4500 feet.
  • Sort by Jim Pollock, reference ibid., page 136. Load $17f5-6 with the LB/HB of the starting address of RAM to sort, $17f7-8 with the ending address + 1, and $021f with $b0 for descending sort or $90 for ascending sort. Starts at $0200. This is a destructive sort.
  • Microchess by Peter Jennings, reference "Microchess: A Chess Playing Program" (self-published). Mega thanks to Paul Foerster for providing this document! This is in five files which should be LOADed in numerical order. You may also optionally load 'microchesso.bin' which is the Giuoco Piano opening for the CPU playing white. Basic play instructions, along with annotated source, are available from the Secret Weapons of Commodore. It is strongly recommended you play in Super-Blitz mode -- in Normal mode on a stock 64, the emulated MicroChess may take up to 30 minutes to compute a move! See the referenced webpage for how to achieve this.

• Tiny BASIC (3K). This self-dissolving archive, when run with a disk in device 8, will dissolve down to two files (TBASIC1.BIN and TBASIC2.BIN) which should be loaded in numerical order. You should load them as "K"IMplement format, not as raw binaries. This is a pre-converted version of Tiny BASIC that the original author, Tom Pittman, offers for download from his Tiny BASIC official home page. Please note that Tom retains the copyright on this version, which he has graciously offered to the community to play with.

  This version is virgin and needs a bit of setup to get running. Here's how:

  • Load both portions into RAM.
  • Switch into TTY mode by pressing RUBOUT (INST/DEL).
  • Type in this sequence (no spaces unless explicitly stated). Be very careful not to miss any keys.

    2006 [space]
    [return]
    5a [period]
    1e [period]
    [return]
    a0 [period]
    1e [period]
    4c [period]
    00 [period]
    01 [period]
    08 [period]
    2000 [space]

    What this is doing is setting the internal jump table to reference the TTY (using the KIM routines at $1ea0 and $1e5a), using Tiny BASIC's TTY break routine to act as break during execution, and setting the backspace key to ASCII 8. (You can't use RUBOUT; it has a special meaning to Tiny BASIC. Get used to hitting CRSR LEFT or RIGHT.)

  • If you think you did this right, type G to start the interpreter (if you want to check, you can see a sample transcript on the right). After around 15 or 20 seconds while the interpreter checks how much RAM there is, a colon prompt will appear.

  You are now in Tiny BASIC. Go read the user manual. RAM is available from $2900 to $3fff for your programs.

  Remember to ALWAYS USE CRSR LEFT or RIGHT to backspace in Tiny BASIC. RUBOUT will appear to work, but Tiny BASIC will complain, and when you list the program you will see that it was intercepted by Tiny BASIC. If you want to use some other character, substitute its ASCII code for 08 above, but KIMplement only knows to move the carriage back for BS and RUBOUT codes. Once you have Tiny BASIC customized the way you want it, you might save a copy to save typing later.

  Break is triggered during program execution with any activity on the TTY (except, of course, in INPUT statements). I like to hold down CRSR RIGHT until the execution stops, and then release it, as this seems to disturb the terminal the least.

  To return to KIM, you can type print usr(7168) or just reset it. If you use the usr() escape, remember that this leaves you in keyBoard mode, so tap RUBOUT again to get the KIM's attention. To warm-return to Tiny BASIC without disturbing your program, start execution at $2003, not $2000.

  Please remember that Tiny BASIC is no speed demon even on a real KIM, and doubly so on KIMplement, due to the fact it itself is written in an interpreted middle level-language. However, this makes it considerably more compact, thus the rationale for doing it that way. It is certainly much smaller than Microsoft BASIC, which doesn't even fit into KIMplement.

• FOCAL-65 (6K). This self-dissolving archive, when run with a disk in device 8, will dissolve down to two files (FOCAL1.BIN and FOCAL2.BIN) which should be loaded in numerical order. You should load them as "K"IMplement format, not as raw binaries. This is the 1977 Aresco version "3D" which is believed to be freely distributable; while there's another version from the 6502 Program Exchange, this is the one I'm most familiar with.

  To start FOCAL, switch to keyBoard mode with RUBOUT and start the interpreter with 2000 [space] g. It will (character by character) display an error code and an asterisk prompt. The border flashes on errors or system messages due to this program's use of the BRK instruction as a software interrupt.

  When you type initially, the terminal will "stutter." Turn off FOCAL's local echo by typing s fech(1) and press RETURN (this will turn into ss ffeecchh((11)) on screen; if you press RUBOUT, you'll see pound signs serving as backslashes). Note that FOCAL "forgets" to keep your echo setting on errors and you may have to type this again after a BRK has occurred. FOCAL also treats the backarrow as delete, so try to avoid using it. However, once you've turned off local echo, RUBOUT (INST/DEL) works as you would expect.

  As well as a functional if somewhat baroque programming language, FOCAL can serve as a simple calculator (type t and an expression) out to a configurable number of decimal places. It is also notable for its "floating point" module-line number format, derived from its ancestor JOSS, as demonstrated at right with the sample program running in the KIMplement computing square roots using Newton's method. Aresco FOCAL-65 adds several utility functions which you can read about in the manual. This version of FOCAL-65 is limited to single character variables (you will get an error using more than one), and exponentiation truncates any decimal portion, though negative numbers are supported if the unary minus is in parentheses, e.g. t 34.67^(-12) from the manual. For example, t 5^0.5 returns 1, the same as t 5^0, but does not throw an error.

  Don't forget erase all to clear the program area, and write to display the interpreter version and any program in memory.

  To exit back to the KIM monitor, just reset it (press the Commodore key twice). A program in memory will be left intact, so you can also halt a program this way: if you call $2000 again after resetting the machine but with FOCAL still in memory, if a program had been running, FOCAL will display where its execution halted.

  FOCAL is even slower than Tiny BASIC due in large part to its heavy floating point usage, but this is also a nice stress test for the emulator and shows that the 6502 was certainly capable of high precision math.

  FOCAL's speed also means that it may not respond immediately to keyboard input (ASK prompts are particularly slow: it may take a second or two to process each character). KIMplement feeds it characters from the C64 Kernal's keyboard buffer and marks the terminal "not idle" so that the main loop will eventually fetch all the keys, but the buffer overflows after 10 characters, so don't type too far ahead!

• paptap.pl. This Perl script takes a KIM-format paper tape file and breaks it apart into binaries. It can optionally add KIMplement starting addresses to make it directly compatible. You need to specify a filename for every non-contiguous section in the tape transcript. As a sample case, try Tiny BASIC in punched tape, which requires two files (look at the punchtape transcript if you don't understand why).

Acknowledgements

Thank-yous go out to: Phil Lange (who did a lot of banging on version 0.1 and offered many suggestions and requests for 0.2), Neil H. F. Meyer, Jr., Cameron Bean, Jim Butterfield, Ruud Baltissen, Peter Jennings and Tom Pittman; apologies to Wendy Carlos and, of course, Chuck Peddle.

The KIM-1 system ROMs appear for educational purposes only, as their contents are no longer considered reasonably available under the fair use/obsolescence criterion 3 of the United States Digital Millenium Copyright Act (reference: http://www.copyright.gov/1201/ and http://www.arstechnica.com/archive/news/1067440261.html). If the authorized owner of the copyrights on these ROMs would like to assert their objection and their proof of copyright on paper, please E-mail me at the address above for contact information. This currently only applies to USA domestic users of the KIMplement; other copyright or fair use laws may apply to international users depending on your home country or region.