Apple II User Manual

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|  TOPIC -- Apple II -- WOZPAK Sweet-16 article by Steve Wozniak

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SWEET 16: A Pseudo 16 Bit Microprocessor

by Steve Wozniak

Description:

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While writing APPLE BASIC for a 6502 microprocessor, I repeatedly

encountered a variant of MURPHY'S LAW. Briefly stated, any routine

operating on 16-bit data will require at least twice the code that

it should. Programs making extensive use of 16-bit pointers (such

as compilers, editors, and assemblers) are included in this

category. In my case, even the addition of a few double-byte

instructions to the 6502 would have only slightly alleviated the

problem. What I really needed was a 6502/RCA 1800 hybrid - an

abundance of 16-bit registers and excellent pointer capability.

My solution was to implement a non-existant (meta) 16-bit

processor in software, interpreter style, which I call SWEET 16.

SWEET 16 is based on sixteen 16-bit registers (R0-15), which are

actually 32 memory locations. R0 doubles as the SWEET 16

accumulator (ACC), R15 as the program counter (PC), and R14 as the

status register. R13 holds compare instruction results and R12 is

the subroutine return stack pointer if SWEET 16 subroutines are

used. All other SWEET 16 registers are at the user's unrestricted

disposal.

SWEET 16 instructions fall into register and non-register categories.

The register ops specify one of the sixteen registers to be used as

either a data element or a pointer to data in memory, depending

on the specific instruction. For example INR R5 uses R5 as data

and ST @R7 uses R7 as a pointer to data in memory. Except for the

SET instruction, register ops take one byte of code each. The

non-register ops are primarily 6502 style branches with the second

byte specifying a +/-127 byte displacement relative to the address

of the following instruction. Providing that the prior register op

result meets a specified branch condition, the displacement is

added to the SWEET 16 PC, effecting a branch.

SWEET 16 is intended as a 6502 enhancement package, not a stand

alone processor. A 6502 program switches to SWEET 16 mode with a

subroutine call and subsequent code is interpreted as SWEET 16

instructions. The nonregister op RTN returns the user program to

6502 mode after restoring the internal register contents

(A, X, Y, P, and S). The following example illustrates how to use

SWEET 16.

300  B9 00 02           LDA   IN,Y     ;get a char

303  C9 CD              CMP   #"M"     ;"M" for move

305  D0 09              BNE   NOMOVE   ;No. Skip move

307  20 89 F6           JSR   SW16     ;Yes, call SWEET 16