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KERNAL[1] isCommodore's name for theROM-residentoperating system core in its8-bithome computers: from the originalPET of 1977, followed by the extended but related versions used in its successors: theVIC-20;Commodore 64;Plus/4;Commodore 16; andCommodore 128.
The Commodore 8-bit machines' KERNAL consists of the low-level, close-to-the-hardware OS routines roughly equivalent to theBIOS in IBM PC compatibles (in contrast to theBASIC interpreter routines, also located in ROM) as well as higher-level, device-independent I/O functionality. It is user-callable via ajump table in RAM whose central (oldest) part, for reasons ofbackwards compatibility,[a] remains largely identical throughout the whole 8-bit series. The KERNAL ROM occupies the last 8KB of the 8-bit CPU's 64 KB address space ($E000–$FFFF).[citation needed]
The jump table can be modified to point to user-written routines, for example, to integrate afast loader so that its fast replacement routines are used system-wide or to replace the system text output routine with one that works in bitmapped mode rather than character mode. This use of a jump table was novel in small computers at the time.[2]
TheAdventure International games published for the VIC-20 on thecartridge are an example of software that uses the KERNAL. Because they only use the jump table, the games can bememory dumped to disk, loaded into a Commodore 64, and run without modification.[3]
The KERNAL was initially written for the Commodore PET by John Feagans, who introduced the idea of separating the BASIC routines from the operating system. It was further developed by several people, notably Robert Russell, who added many of the features for the VIC-20 and the C64.[citation needed]
A simple, yet characteristic, example of using the KERNAL is given by the following6502assembly language subroutine[b] (written inca65 assembler format/syntax), implementing the classic "Hello, world!" program:
CHROUT=$ffd2; CHROUT is the address of the character output routineCR=$0d; PETSCII code for Carriage Return;hello:ldx#0; start with character 0 by loading 0 into the x index registernext:ldamessage,x; load byte from address message+x into the accumulatorbeqdone; if the accumulator holds zero, we're done and want to branch out of the loopjsrCHROUT; call CHROUT to output char to current output device (defaults to screen)inx; increment x to move to the next characterbnenext; loop back while the last character is not zero (max string length 255 bytes)done:rts; return from subroutine;message:.byte"Hello, world!".byteCR,0; Carriage Return and zero marking end of string
Note the use of thePETSCII code forCarriage Return, the xindex register, and theaccumulator. This code stub employs theCHROUT routine, whose address is found at address$FFD2 (65490), to send a text string to the default output device (e.g., the display screen).[citation needed]
The KERNAL was known askernel[c] inside of Commodore since the PET days, but in 1980 Robert Russell misspelled the word askernal in his notebooks. When Commodore technical writers Neil Harris and Andy Finkel collected Russell's notes and used them as the basis for the VIC-20 programmer's manual, the misspelling followed them along and stuck.[4]
According to early Commodore myth, and reported by writer/programmerJim Butterfield among others, the "word" KERNAL is anacronym (or, more likely, abackronym) standing forKeyboardEntryRead,Network,AndLink, which is at least somewhat credible considering its role.Berkeley Softworks later used it when naming the core routines of itsGUI OS for 8-bit home computers: theGEOS KERNAL.[citation needed]
Surprisingly, the KERNAL implemented a device-independent I/O API not entirely dissimilar from that ofUnix orPlan-9, which nobody actually exploited, as far as is publicly known. Whereas one could reasonably argue that "everything is a file" in these latter systems, others could easily claim that "everything is aGPIB-device" in the former.[citation needed]
Due to limitations with the 6502 architecture at the time, opening an I/O channel requires threesystem calls. The first typically sets the logical filename through theSETNAM system call. The second call,SETLFS, establishes the GPIB/IEEE-488 "device" address to communicate with. FinallyOPEN is called to perform the actual transaction. The application then usedCHKIN andCHKOUT system calls to set the application's current input and output channels, respectively. Applications may have any number of concurrently open files (up to some system-dependent limit; e.g., the C64 allows for ten files to be opened at once). Thereafter,CHRIN andCHROUT prove useful for actually conducting input and output, respectively.CLOSE then closes a channel.[citation needed]
Observe that no system call exists to "create" an I/O channel, for devices cannot be created or destroyed dynamically under normal circumstances. Likewise, no means exists for seeking, nor for performing "I/O control" functions such asioctl() in Unix. Indeed, the KERNAL proves much closer to the Plan-9 philosophy here, where an application would open a special "command" channel to the indicated device to conduct such "meta" or "out-of-band" transactions. For example, to delete ("scratch") a file from a disk, the user typically will "open" the resource calledS0:THE-FILE-TO-RMV on device 8 or 9, channel 15. Per established convention in the Commodore 8-bit world, channel 15 represents the "command channel" for peripherals, relying on message-passing techniques to communicate both commands and results, including exceptional cases. For example, inCommodore BASIC, they might find software not unlike the following:[citation needed]
70...80REM ROTATE LOGS CURRENTLY OPENED ON LOGICAL CHANNEL #1.90CLOSE1100OPEN15,8,15,"R0:ERROR.1=0:ERROR.0":REM RENAME FILE ERROR.0 TO ERROR.1110INPUT#15,A,B$,C,D:REM READ ERROR CHANNEL120CLOSE15130IFA=0THENGOTO200140PRINT"ERROR RENAMING LOG FILE:"150PRINT" CODE: ";A160PRINT" MSG : "+B$170END200REM CONTINUE PROCESSING HERE, CREATING NEW LOG FILE AS WE GO...210OPEN1,8,1,"0:ERROR.0,S,W"220...
Device numbers, per established documentation, are restricted to the range [0,16]. However, this limitation came from the specific adaptation of the IEEE-488 protocol and, in effect, applies only to external peripherals. With all relevant KERNAL system calls vectored, programmers can intercept system calls to implement virtual devices with any address in the range of [32,256]. Conceivably, one can load adevice driver binary into memory, patch the KERNAL I/O vectors, and from that moment forward, a new (virtual) device could be addressed. So far, this capability has never been publicly known as utilized, presumably for two reasons: (1) The KERNAL provides no means for dynamically allocating device IDs, and (2) the KERNAL provides no means for loading a relocatable binary image. Thus, the burden of collisions both in I/O space and in memory space falls upon the user, while platform compatibility across a wide range of machines falls upon the software author. Nonetheless, support software for these functions could easily be implemented if desired.[citation needed]
Logical filename formats tends to depend upon the specific device addressed. The most common device used, of course, is the floppy disk system, which uses a format similar toMD:NAME,ATTRS, where M is a flag of sorts ($ for directory listing, @ for indicating a desire to overwrite a file if it already exists, unused otherwise.), D is the (optional) physical disk unit number (0: or 1: for dual-drive systems, just 0: for single-disk units like the 1541, et al., which defaults to 0: if left unspecified),NAME is a resource name up to 16 characters in length (most characters allowed except for certain special characters), andATTRS is an optional comma-separated list of attributes or flags. For example, if the user wants to overwrite a program file calledPRGFILE, they might see a filename like@0:PRGFILE,P used in conjunction with device 8 or 9. Meanwhile, a filename for theRS-232 driver (device 2) consists simply of four characters, encoded in binary format.[5]
Other devices, such as the keyboard (device 0), cassette (device 1), the display interface (device 3), and printer (device 4 and 5), require no filenames to function, either assuming reasonable defaults or simply not needing them at all.[citation needed]