A SWEET16 inspired 32-bit VM for the 65816 (SW32 for short)
This CA65 Library implements a 32-bit RISC Processor in software for the 65816, which can then be used to more easily handle 32-bit functions (or if you're just more familiar with RISC-like Instruction sets) It has the following features:
- 15x 32-bit General Purpose Registers R1-R15 (R0 is a constant 0)
- 32-bit Program Counter (though functionally it's only 24-bits wide)
- 8-bit Status Register called SR, with 3 ALU FLags (Zero, Carry, Negative), and 4 user programmable Flags F4-F7
- 24-bit Addressing for both Program flow and Load/Store Instructions
Almost all Instructions are 2 Bytes long, except for the "Load Immediate" Instructions which are either 4 or 6 Bytes in length.
It should be as simple as including the SWEET32.inc file in your main source file, which defines all the macros for the VM's Instruction set and include the SWEET32.lib file when Linking with ld65 (just make sure it's located after (ie to the right) your source files in the ld65 command)
in case you need/want to modify the stock VM, the source files are simple enough to assemble and combine into a .lib file like this:
CA65 -v --cpu 65816 -o sw32_init.o sw32_init.a816
CA65 -v --cpu 65816 -o sw32_execute.o sw32_execute.a816
CA65 -v --cpu 65816 -o sw32_print.o sw32_print.a816
AR65 v v r SWEET32.lib sw32_init.o sw32_execute.o sw32_print.o
The Format is pretty simple: Ra = Source Register A Rb = Source Register B Re = Destination Register Rx = Source and Destination Register
| Name | Mnemonic | Description |
|---|---|---|
| Branch on Clear | BNv k | Branches if the specified bit "v" in the SR is 0 "k" is an 8-bit signed offset multiplied by 2, giving it a -128 to +127 Word range |
| Branch on Set | Bv k | Branches if the specified bit "v" in the SR is 1 (Offset is the same as above) |
| Jump and Link | JAL Re, Ra | Jumps to the Address in the Source Register and stores the address of the following instruction in the Destination Register using R0 as the Destination Register makes JAL function like a regular Jump, using R0 as the Source Register won't modify the PC, bascially just loading the Address of the next Instruction into a Register |
| Add | ADR Re, Ra, Rb | Re = Ra + Rb, updated Flags: Z, C, N |
| Subtract | SBR Re, Ra, Rb | Re = Ra - Rb, updated Flags: Z, C, N |
| Logic AND | ANR Re, Ra, Rb | Re = Ra AND Rb, updated Flags: Z, N |
| Logic OR | ORR Re, Ra, Rb | Re = Ra OR Rb, updated Flags: Z, N |
| Logic XOR | XOR Re, Ra, Rb | Re = Ra XOR Rb, updated Flags: Z, N |
| Logic Shift Left | SFL Re, Ra | Re = C <- Ra <- 0, updated Flags: Z, C, N |
| Logic Shift Right | SFR Re, Ra | Re = 0 -> Ra -> C, updated Flags: Z, C, N |
| Rotate Left | RLL Re, Ra | Re = C <- Ra <- C, updated Flags: Z, C, N |
| Rotate Right | RRR Re, Ra | Re = C -> Ra -> C, updated Flags: Z, C, N |
| Add Immediate | ADI Rx, k | Rx = Rx + k (8-bit, sign-extended to 32-bit), updated Flags: Z, C, N |
| Load Byte (Signed) | LB Re, Ra | Uses the contents of the Source Register as an Address to load a Byte, sign-extend it, and store it into the Destination Register |
| Load Byte (Unsigned) | LBU Re, Ra | Same as above, except it zero-extends the Byte |
| Load Word (Signed) | LW Re, Ra | Same as the first, except it loads a 16-bit word and sign-extends it |
| Load Word (Unsigned) | LWU Re, Ra | Same as the first, except it loads a 16-bit word and zero-extends it |
| Load Long | LL Re, Ra | Same as the first, except it loads a full 32-bit word, so no "extending" is required |
| Store Byte | SB Ra, Rb | Uses the contents of the Source Register A as an Address, and stores the Low Byte of the Source Register B to that Address |
| Store Word | SW Ra, Rb | Same as above, except it stores the Low Word of the Source Register B |
| Store Long | SL Ra, Rb | Same as the first, except it stores the whole Source Register B |
| Set Bit | SET Rx, k | Sets the "k"th bit in the specified Register, if the Register is R0 "k" is instead used as an 8-bit wide mask to select which bits in the SR should be set |
| Clear Bit | CLR Rx, k | Clears the "k"th bit in the specified Register, if the Register is R0 "k" is instead used as an 8-bit wide mask to select which bits in the SR should be cleared |
| Load Word Immediate (Signed) | LWI Re, k | Sign-extends the 16-bit constant "k" to 32-bits and loads that value into the Destination Register |
| Load Word Immediate (Unsigned) | LWIU Re, k | Same as above, except it zero-extends the 16-bit constant |
| Load Long Immediate | LLI Re, k | Same as the first, except the constant is 32-bit wide and therefore doesn't require "extending" |
| Return to 65816 Mode | EXIT | Exits the VM and resumes regular 65816 execution after the sw32_execute function |
some more detail about the Flags:
the Zero Flag (Z) is set when the result of an operation is equal to 0
the Negative FLag (N) is a copy of the MSB (bit 31) of the result of an operation
the Carry Flag (C) depends on the operation:
- Add/Add Immediate sets the Carry when the result overflowed (overflew?)
- Subtract sets the Carry when the result went below 0 (like an overflow but in the opposite direction)
- Shifts/Rotates set the Carry to the value of the bit that was shifted out, in addition Rotates shift the previous Carry into the number as well
There are 2 main functions required to make the VM Work, and 3 extra functions for printing (ALL of them are called with JSL, and expect 8-bit A, and 16-bit X/Y)
sw32_init - clears all Registers, and sets the Control Byte to the value in A
Currently the Control byte's only used bit is bit 7, which determins the address width for Load/Store Instructions.
if bit 7 is cleared Load/Store Instructions are limited to 16-bit addressing, if set they are 24-bit instead.
sw32_execute - executes SW32 code, it starts at the address given by the X and Y Registers. X = Low Word, Y = High Word (High Byte is ignored)
The function only returns when an EXIT instruction is executed.
sw32_print - prints out the contents of all Registers in a nice and tidy format:
R1: $00000000
R2: $00000000
R3: $00000000
R4: $00000000
R5: $00000000
R6: $00000000
R7: $00000000
R8: $00000000
R9: $00000000
R10: $00000000
R11: $00000000
R12: $00000000 PC: $00000000
R13: $00000000
R14: $00000000 7654NCZ
R15: $00000000 SR: 0000000
sw32_print_h8 - Prints the 8-bit value in A
sw32_print_h32 - Prints the 32-bit value that was pushed to the stack before calling the function (push the high word first)
in order for the print functions to work they needs a user provided function to print a single ASCII Character to whatever output the user might have said function has to be called "sw32_print_char", return with RTL and has to assume A is 8-bits wide, and X/Y are 16-bits wide.
btw, There is no "sw32_print_h16" function, so if needed the user has to implement their own.