SWEET16
Encyclopedia
SWEET16 is an interpreted "byte-code" language invented by Steve Wozniak
and implemented as part of the Integer BASIC
ROM
in the Apple II series
of computers. It was created because Wozniak needed to manipulate 16-bit
pointer data in his implementation of BASIC
, and the Apple II was an 8-bit computer.
SWEET16 code is executed as if it were running on a 16-bit processor
with sixteen internal 16-bit little-endian
registers
, named R0 through R15. Some registers have well-defined functions:
The 16 virtual registers, 32 bytes in total, are located in the zero page
of the Apple II's real, physical memory map (at $00–$1F), with values stored as low byte followed by high byte. The SWEET16 interpreter itself is located from $F689 to $F7FC in the Integer BASIC
ROM.
According to Wozniak, the SWEET16 implementation is a model of frugal coding
, taking up only about 300 byte
s in memory. SWEET16 runs about one-tenth the speed of the equivalent native 6502
code.
Steve Wozniak
Stephen Gary "Woz" Wozniak is an American computer engineer and programmer who founded Apple Computer, Co. with Steve Jobs and Ronald Wayne...
and implemented as part of the Integer BASIC
Integer BASIC
Integer BASIC, written by Steve Wozniak, was the BASIC interpreter of the Apple I and original Apple II computers. Originally available on cassette, then included in ROM on the original Apple II computer at release in 1977, it was the first version of BASIC used by many early home computer owners...
ROM
Read-only memory
Read-only memory is a class of storage medium used in computers and other electronic devices. Data stored in ROM cannot be modified, or can be modified only slowly or with difficulty, so it is mainly used to distribute firmware .In its strictest sense, ROM refers only...
in the Apple II series
Apple II series
The Apple II series is a set of 8-bit home computers, one of the first highly successful mass-produced microcomputer products, designed primarily by Steve Wozniak, manufactured by Apple Computer and introduced in 1977 with the original Apple II...
of computers. It was created because Wozniak needed to manipulate 16-bit
Bit
A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...
pointer data in his implementation of BASIC
BASIC
BASIC is a family of general-purpose, high-level programming languages whose design philosophy emphasizes ease of use - the name is an acronym from Beginner's All-purpose Symbolic Instruction Code....
, and the Apple II was an 8-bit computer.
SWEET16 code is executed as if it were running on a 16-bit processor
Central processing unit
The central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...
with sixteen internal 16-bit little-endian
Endianness
In computing, the term endian or endianness refers to the ordering of individually addressable sub-components within the representation of a larger data item as stored in external memory . Each sub-component in the representation has a unique degree of significance, like the place value of digits...
registers
Processor register
In computer architecture, a processor register is a small amount of storage available as part of a CPU or other digital processor. Such registers are addressed by mechanisms other than main memory and can be accessed more quickly...
, named R0 through R15. Some registers have well-defined functions:
- R0 is the accumulatorAccumulator (computing)In a computer's central processing unit , an accumulator is a register in which intermediate arithmetic and logic results are stored. Without a register like an accumulator, it would be necessary to write the result of each calculation to main memory, perhaps only to be read right back again for...
. - R12 is the subroutine stack pointer.
- R13 stores the result of all comparison operations for branchBranch (computer science)A branch is sequence of code in a computer program which is conditionally executed depending on whether the flow of control is altered or not . The term can be used when referring to programs in high level languages as well as program written in machine code or assembly language...
testing. - R14 is the status registerStatus registerA status register or flag register is a collection of flag bits for a processor. An example is the FLAGS register of the x86 architecture....
. - R15 is the program counterProgram counterThe program counter , commonly called the instruction pointer in Intel x86 microprocessors, and sometimes called the instruction address register, or just part of the instruction sequencer in some computers, is a processor register that indicates where the computer is in its instruction sequence...
.
The 16 virtual registers, 32 bytes in total, are located in the zero page
Zero page
The zero page is the series of memory addresses at the absolute beginning of a computer's address space; that is, the page whose starting address is zero. The size of a "page" depends on the context, and the significance of zero-page memory versus higher addressed memory is highly dependent on...
of the Apple II's real, physical memory map (at $00–$1F), with values stored as low byte followed by high byte. The SWEET16 interpreter itself is located from $F689 to $F7FC in the Integer BASIC
Integer BASIC
Integer BASIC, written by Steve Wozniak, was the BASIC interpreter of the Apple I and original Apple II computers. Originally available on cassette, then included in ROM on the original Apple II computer at release in 1977, it was the first version of BASIC used by many early home computer owners...
ROM.
According to Wozniak, the SWEET16 implementation is a model of frugal coding
Computer programming
Computer programming is the process of designing, writing, testing, debugging, and maintaining the source code of computer programs. This source code is written in one or more programming languages. The purpose of programming is to create a program that performs specific operations or exhibits a...
, taking up only about 300 byte
Byte
The byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...
s in memory. SWEET16 runs about one-tenth the speed of the equivalent native 6502
MOS Technology 6502
The MOS Technology 6502 is an 8-bit microprocessor that was designed by Chuck Peddle and Bill Mensch for MOS Technology in 1975. When it was introduced, it was the least expensive full-featured microprocessor on the market by a considerable margin, costing less than one-sixth the price of...
code.
External links
- Call-A.P.P.L.E. Wozpak II — 1979 Call-A.P.P.L.E.Call-A.P.P.L.E.Call-A.P.P.L.E. Magazine is the monthly journal publication of the Apple Pugetsound Program Library Exchange . The magazine was published from 1978 until 1990 when it was discontinued; after a 12 year lapse publication was restarted in 2002...
booklet that includes "SWEET 16 Introduction" by Dick Sedgewick and a version of "SWEET 16: The 6502 Dream Machine" by Steve WozniakSteve WozniakStephen Gary "Woz" Wozniak is an American computer engineer and programmer who founded Apple Computer, Co. with Steve Jobs and Ronald Wayne...
with longer descriptions of each opcodeOpcodeIn computer science engineering, an opcode is the portion of a machine language instruction that specifies the operation to be performed. Their specification and format are laid out in the instruction set architecture of the processor in question...