View source for Sweet16

This is a masterpiece of 6502 programming as far as I'm concerned [[User:Lucky|Lucky]] 19:06, 17 May 2007 (PDT)
{{wp-orig}}
'''SWEET16''' is an interpreted byte-code language invented by Steve Wozniak and implemented as part of the [[Integer BASIC]] ROM in the Apple II computer.  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 (non-existent) 16-bit processor with sixteen, internal 16-bit little-endian registers, R0 through R15.  Some registers have well-defined functions:

* R0 is the accumulator.
* R14 is the status register.
* R13 stores the result of all comparison operations for branch testing.
* R15 is the program counter.

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). The actual SWEET16 interpreter is located from $F689 to $F7FC.

According to Wozniak, the SWEET16 implementation is a model of frugal coding, taking up only about 300 bytes in memory. SWEET16 runs about 10 times slower than the equivalent native [[MOS Technology 6502|6502]].

The following table is from Byte Magazine, November 1977:

{| class="wikitable"
|-
! colspan="8"| SWEET16  OPCODE  SUMMARY
|-
! colspan="4"| Register Ops 
! colspan="4"| 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 indirect)
| 06 || BZ  || ea  || (Branch if Zero)
|-
| 7n || STD || @Rn || (Store double 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)
| OA || BK  || ea  || (Break)
|-
| Bn || SUB  || Rn || (Sub)
| OB || RS   ||    || (Return from Subroutine)
|-
| Cn || POPD || @Rn || (Pop double indirect)
| 0C || BS   || ea  || (Branch to Subroutine)
|-
| Dn || CPR  || Rn  || (Compare)
| 0D ||      ||     || (Unassigned)
|-
| En || INA  || Rn || (Increment)
| OE ||      ||    || (Unassigned)
|-
| Fn || DCR  || Rn || (Decrement)
| OF ||      ||    || (Unassigned)
|-
|}

== External Links ==

* [http://www.6502.org/source/interpreters/sweet16.htm An extensive discussion of the interpreter and source code]