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VM implementations for the RW instructions set architecture
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README.md

RW ISA VM examples

Example imlementations Virtual Machines (VMs) for the "RW" Instruction Set Architecture.

These VMs can be used to run the example executables from http://rolfwr.net/rwisa/rwisa-exe.zip

Companion presentation slides at http://rolfwr.net/tarpit/

The RW instuction set can be summarized as:

Opcode Description C-like implementation
00 Halt exit(0)
01 srcptr Output Byte putchar(mem[srcptr])
02 jmpptr srcptr Branch If Plus if (mem[srcptr] < 128) pc = jmpptr
03 dstptr srcptr Subtract mem[dstptr] -= mem[srcptr]
04 dstptr Input Byte mem[dstptr] = getchar()

The trailing bytes following an opcode byte indicated by srcptr, dstptr and jmpptr represent little endian encoded unsigned integer values that refer to memory locations. For ISA identifiers suffixed by "2" such as RWa2 and RWb2, the these are four byte little endian unsigned integers. I.e. this format uses 32-bit pointers.

The RWa2 executable image is headerless, meaning that the first byte of an executable image contains the first opcode to be executed.

The RWb executable image has a simple header which which allows for both a .text section for code and data, and a .bss section for zero initialized working memory.

The RWb executable image format can be summarized as follows:

Offset Size What
0 2 Magic 'RW'
2 1 ISA revision + 97
3 1 Pointer size (ps): Log2(ps) + 48
4 ps End of file (eof) pointer, .BSS segment start
4+ps ps End of memory (eom) pointer, .BSS segment end
4+2×ps ps .TEXT segment, executable code and data
eof eom-eof Zero initialized runtime memory (.BSS)

The executable image file format and the runtime memory layout are identical, with the exception that the last part consisting of the zero initialized runtime memory .BSS segment is not stored as part of the file.

By convention, executable image files with headers are given a file name extension that matches the lower case ASCII repesentation of the first four bytes of the file. E.g. an image file with ISA revision 2 and pointer size 4 is has extension ".rwb2", and if we change the pointer size to 8, then the extension becomes ".rwb3". Headerless executable images implies that the ISA revision is 1, and the pointer size is 4. These executable image files are given the extension ".rwa2" by convention.

The following ISA revisions have been defined:

Rev# Description
1 Executable image without header, opcodes 0–4 used.
2 Executable image with header, opcodes 0–4 used.
3 Executable image with header, opcodes 0–7 used.

The instructions of opcodes 5–7 defined by revision 3 can be synthesized using existing instructions, but allow for faster execution when used as distinct opcodes.

Opcode Description C-like implementation
05 dstptr srcptr Move Byte mem[dstptr] = mem[srcptr]
06 jmpptr srcptr Branch If Zero if (mem[srcptr] == 0) pc = jmpptr
07 dstptr srcptr Add Pointers writeptr(mem, dstptr) = readptr(mem, dstptr) + readptr(mem.srcptr)
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