A terrible idea: JZFORTH

• A FORTH for ARM Jazelle
  • it will be inspired by jonesforth
• A handcrafted ELF
  • a few pages of ARM code for initialization and opcode handlers
    • a handler for syscall instructions
  • "JVM" code for the static forth dictionary
    • assembled with a jvm assembler
    • implements a runtime forth compiler
    • special functionality to write the current process to a file and to undefine words / free memory

Implementation details

• Syscalls use the impdep1 JVM instruction
  • fe 01 syscall1 arg0, sysnr -> ret
  • fe 00 syscall0 sysnr -> ret
• Memory layout
  • Base image
    • ELF header and Program header
      • Forth entry word in e_ident
    • ARM initialization code
    • hello world in jvm code
    • basic forth dictionary in jvm code
  • Stack
    • Jazelle handler table
    • Locals table
• JVM locals: important forth state
  • space for temporaries
  • base image start and end
  • return address
  • return stack pointer
• calls
  • call: jsr foo
  • leaf functions
    • prologue: astore #5 - save RA to local
    • epilogue: ret #5
  • functions that call other functions
    • prologue: push RA to return stack
    • epilogue: pop RA from return stack, write to local, ret

Hello world

A hello world in JVM assembly: (the numbers on the left are the current stack depth)

        // write the well-known message
        .byte 0x10, 13                  // 0  bipush length
        .byte 0x1a                      // 1  iload_0 = load BASE from locals
        .byte 0x10, 0x10                // 2  bipush offset of string
        .byte 0x60                      // 3  iadd
        .byte 0x04                      // 2  iconst_1 = stdout
        .byte 0x07                      // 3  iconst_4 = SYS_write
        .byte 0xfe, 3                   // 4  syscall3 (fe 03)
        .byte 0x57                      // 1  pop

        // exit
        .byte 0x10, 42                  // 0  bipush 42
        .byte 0x04                      // 1  iconst_1 = SYS_exit
        .byte 0xfe, 1                   // 2  syscall1 (fe 01)

        .ascii  "Hello World!\n"

References

• Jonesforth (GitHub mirror)
• A Whirlwind Tutorial on Creating Really Teensy ELF Executables for Linux