SWEET16
Template:Short description Script error: No such module "other uses". 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 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 <templatestyles src="Mono/styles.css" />R0 through <templatestyles src="Mono/styles.css" />R15. Some registers have well-defined functions:[1]
- <templatestyles src="Mono/styles.css" />R0 – accumulator
- <templatestyles src="Mono/styles.css" />R12 – subroutine stack pointer
- <templatestyles src="Mono/styles.css" />R13 – stores the result of all comparison operations for branch testing
- <templatestyles src="Mono/styles.css" />R14 – status register
- <templatestyles src="Mono/styles.css" />R15 – 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 <templatestyles src="Mono/styles.css" />$00–<templatestyles src="Mono/styles.css" />$1F), with values stored as low byte followed by high byte.[1] The SWEET16 interpreter itself is located from <templatestyles src="Mono/styles.css" />$F689 to <templatestyles src="Mono/styles.css" />$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. SWEET16 runs at about one-tenth the speed of the equivalent native 6502 code; however it shaved around 20% off the size of Integer BASIC.[3]
Operators
| Register Ops | Nonregister Ops | ||||
|---|---|---|---|---|---|
| 00 | <templatestyles src="Mono/styles.css" />RTN | Return to 6502 mode | |||
| 1n | <templatestyles src="Mono/styles.css" />SET Rn | Constant set | 01 | <templatestyles src="Mono/styles.css" />BR ea | Branch always |
| 2n | <templatestyles src="Mono/styles.css" />LD Rn | Load | 02 | <templatestyles src="Mono/styles.css" />BNC ea | Branch if No Carry |
| 3n | <templatestyles src="Mono/styles.css" />ST Rn | Store | 03 | <templatestyles src="Mono/styles.css" />BC ea | Branch if Carry |
| 4n | <templatestyles src="Mono/styles.css" />LD @Rn | Load indirect | 04 | <templatestyles src="Mono/styles.css" />BP ea | Branch if Plus |
| 5n | <templatestyles src="Mono/styles.css" />ST @Rn | Store indirect | 05 | <templatestyles src="Mono/styles.css" />BM ea | Branch if Minus |
| 6n | <templatestyles src="Mono/styles.css" />LDD @Rn | Load double-byte indirect | 06 | <templatestyles src="Mono/styles.css" />BZ ea | Branch if Zero |
| 7n | <templatestyles src="Mono/styles.css" />STD @Rn | Store double-byte indirect | 07 | <templatestyles src="Mono/styles.css" />BNZ ea | Branch if NonZero |
| 8n | <templatestyles src="Mono/styles.css" />POP @Rn | Pop indirect | 08 | <templatestyles src="Mono/styles.css" />BM1 ea | Branch if Minus 1 |
| 9n | <templatestyles src="Mono/styles.css" />STP @Rn | Store Pop indirect | 09 | <templatestyles src="Mono/styles.css" />BNM1 ea | Branch if Not Minus 1 |
| An | <templatestyles src="Mono/styles.css" />ADD Rn | Add | 0A | <templatestyles src="Mono/styles.css" />BK | Break |
| Bn | <templatestyles src="Mono/styles.css" />SUB Rn | Subtract | 0B | <templatestyles src="Mono/styles.css" />RS | Return from Subroutine |
| Cn | <templatestyles src="Mono/styles.css" />POPD @Rn | Pop double-byte indirect | 0C | <templatestyles src="Mono/styles.css" />BS ea | Branch to Subroutine |
| Dn | <templatestyles src="Mono/styles.css" />CPR Rn | Compare | 0D | Unassigned | |
| En | <templatestyles src="Mono/styles.css" />INR Rn | Increment | 0E | Unassigned | |
| Fn | <templatestyles src="Mono/styles.css" />DCR Rn | Decrement | 0F | Unassigned | |
References
<templatestyles src="Reflist/styles.css" />
Script error: No such module "Check for unknown parameters".
External links
- Call-A.P.P.L.E. Wozpak II — 1979 Call-A.P.P.L.E. booklet that includes "SWEET 16 Introduction" by Dick Sedgewick and a version of "SWEET 16: The 6502 Dream Machine" by Steve Wozniak with longer descriptions of each opcode
- Script error: No such module "citation/CS1".