KERNAL is Commodore's name for the ROM-resident operating system core in its 8-bit home computers: from the original PET of 1977, followed by the extended but related versions used in its successors: the VIC-20; Commodore 64; Plus/4; Commodore 16; and Commodore 128.
Description The Commodore 8-bit machines' KERNAL consists of the low-level, close-to-the-hardware OS routines roughly equivalent to the BIOS in IBM PC compatibles (in contrast to the BASIC interpreter routines, also located in ROM) as well as higher-level, device-independent I/O functionality. It is user-callable via a jump table in RAM whose central (oldest) part, for reasons of backwards compatibility, remains largely identical throughout the whole 8-bit series. The KERNAL ROM occupies the last 8 KB of the 8-bit CPU's 64 KB address space ($E000–$FFFF). The jump table can be modified to point to user-written routines, for example, to integrate a fast 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. The Adventure International games published for the VIC-20 on the cartridge are an example of software that uses the KERNAL. Because they only use the jump table, the games can be memory dumped to disk, loaded into a Commodore 64, and run without modification. 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.
Example A simple, yet characteristic, example of using the KERNAL is given by the following 6502 assembly language subroutine (written in ca65 assembler format/syntax), implementing the classic "Hello, world!" program:
Note the use of the PETSCII code for Carriage Return, the x index register, and the accumulator. This code stub employs the CHROUT routine, whose address is found at address $FFD2 (65490), to send a text string to the default output device (e.g., the display screen).
The name The KERNAL was known as kernel inside of Commodore since the PET days, but in 1980 Robert Russell misspelled the word as kernal 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. According to early Commodore myth, and reported by writer/programmer Jim Butterfield among others, the "word" KERNAL is an acronym (or, more likely, a backronym) standing for Keyboard Entry Read, Network, And Link, which is at least somewhat credible considering its role. Berkeley Softworks later used it when naming the core routines of its GUI OS for 8-bit home computers: the GEOS KERNAL.
On device-independent I/O Surprisingly, the KERNAL implemented a device-independent I/O API not entirely dissimilar from that of Unix or Plan-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 a GPIB-device" in the former. Due to limitations with the 6502 architecture at the time, opening an I/O channel requires three system calls. The first typically sets the logical filename through the SETNAM system call. The second call, SETLFS, establishes the GPIB/IEEE-488 "device" address to communicate with. Finally OPEN is called to perform the actual transaction. The application then used CHKIN and CHKOUT 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 and CHROUT prove useful for actually conducting input and output, respectively. CLOSE then closes a channel. 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 as ioctl() 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 called S0: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, in Commodore BASIC, they might find software not unlike the following:
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