SWEET16

SWEET16 is an interpreted byte-code instruction set 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, and the Apple II was an 8-bit computer.[1]

SWEET16 was not used by the core BASIC code, but was later used to implement several utilities. Notable among these was the line renumbering routine, which was included in the Programmer's Aid #1 ROM, added to later Apple II models and available for user installation on earlier examples.[2]

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:[1]

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.[1] 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 bytes in memory.[3] SWEET16 runs at about one-tenth the speed of the equivalent native 6502 code.[1]

Operators
Register Ops Nonregister Ops
00 RTN Return to 6502 mode
1n SET Rn Constant set 01 BR ea Branch always
2n LD Rn Load 02 BNC ea Branch if No Carry
3n ST Rn Store 03 BC ea Branch if Carry
4n LD @Rn Load indirect 04 BP ea Branch if Plus
5n ST @Rn Store indirect 05 BM ea Branch if Minus
6n LDD @Rn Load double-byte indirect 06 BZ ea Branch if Zero
7n STD @Rn Store double-byte indirect 07 BNZ ea Branch if NonZero
8n POP @Rn Pop indirect 08 BM1 ea Branch if Minus 1
9n STP @Rn Store Pop indirect 09 BNM1 ea Branch if Not Minus 1
An ADD Rn Add 0A BK Break
Bn SUB Rn Subtract 0B RS Return from Subroutine
Cn POPD @Rn Pop double-byte indirect 0C BS ea Branch to Subroutine
Dn CPR Rn Compare 0D Unassigned
En INR Rn Increment 0E Unassigned
Fn DCR Rn Decrement 0F Unassigned

References
  1. Wozniak, Stephen (November 1977). "SWEET16: The 6502 Dream Machine". Byte. Retrieved 2011-01-05.
  2. Apple Programmers Aid (PDF). Apple. 1978.
  3. Wozniak, Stephen (May 1977). "The Apple II" (PDF). Byte. Retrieved 2012-05-18.
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