memxFORTH-asm

This project builds memxFORTH directly from assembler and try to be memory-effective

Target is ATmega2560 at MegaHomeFORTH board

• Normal state is:

  • 8kB RAM used for FORTH Stack and Return Stack, also for System Stack (so these are accessible anytime)
  • Extended Memory is mapped on high part of 128kB and active (so paged RAM is lower 72kB = 64+8)
  • Shared Memory is not accessible

• All pointers should be 3-bytes/24-bits long, also CELL size is 3-bytes, so there is no size mismatch between address and value

  • Extra-byte (bits 16..23) is 0x80 for RAM, 00..03 for FLASH (and maybe something else for paged RAM and Shared Ram)

    • bits:

      • bit 7: 1=RAM; 0=other (special read)

      • bits 76: 00,11 = NOT pointer (=large negative/small positive value), 10, 01 =probabely pointer somewhere, translating it to FORTH may make sense :)

        • this is for SHOW heuristicks to distinquish normal values from FORTH words addresses

      • all FORTH functions for FLASH address should set bit 22 high
      • bits 5..3: reserved (zero now)
      • bit 2: 1= 128kB chips
      • bits 1,0: bank for FLASH, for RAM 0x=external RAM, 1x=Shared RAM

    • 80 - Normal RAM (64 kB) ( ld r24,Z+ style) (last 56kB is top of Extended memory too)
    • 40 - Normal flash (0-64kB)( lpm r24,Z+ style)
    • 41..43 - Normal flash (64-256kB) ( out RAMPZ,??; elpm r24,Z+ style) probably better for 40 too (just 1 clock more) (&46=40)
    • 44..45 - Extended memory (128kB) last 56kB is top of System RAM too (&46=44)
    • 46..47 - Shared memory (128kB) (&46=46)

  • all memory acceess should be via system calls: B1at, B3at, W1at, W3at, THIS and THAT (and maintain Normal state)

    • the access MAY fail if given bad pointer, then it returns (FLAG T) and data are (UNDEFINED) (or it jumps to Bad Memory Handler ???)

      • namely it immediatelly fails when Shared Memory is asked and not allowed by GLUE
      • SET/CLT to set/clear T, BRTS/BRTC branch if T is set/clear

    • extra memory access is via other system calls

• reserved/assigned registers are this, every primitive is requered to protect and update them responsibly (just idea):

  • X+r0 - contain DT after NEXT, discartable
  • Y+r2 - IP (preserved in C)
  • Z - free to use (ELPM, EIJMP ...)
  • SP - ASM RETURN stack
  • r0 (DT)
  • r1 ZERO
  • r2 IP
  • r3..5 TOS - Top Of Stack
  • r6..7 S - Stack (in RAM 0)
  • r8..9 RS - Return Stack (in RAM 0)
  • r10..r17 Free to assign (or preserve before use)
  • r18..r27 Free to use (discartable) (including X=r26..27)
  • r26..27 X - filled with DT in NEXT, discartable
  • r28..29 Y - IP (preserved by C, used by FORTH)
  • r30..31 Z - discartable (used with r25 as Z_hlo)
  • r22..24 temp registers, arguments in and out ...
  • r18..r21 scratch registers for everything and nothing - destroyed over calls

• special registers:

  • Z - EIJMP, ELPM, Z+displacement
  • Y - Y+displacement
  • X - normal ( each can predecrement and postincrement)

• RAMPZ - not preserved by subroutines, set it everytime

#define JMP_ADDR(addr16)                   \
        do {                               \
                __asm__ __volatile__ (     \
                        ".word 0x940C\n\t" \
                        ".word %0\n\t"     \
                        : : "i" (addr16)   \
                );                         \
    } while (0)

register uint8_t counter asm("r2");  // r2 is the counter
__attribute__((naked, noreturn)) void function_A(void) { ... ;EIJMP; }

__attribute__((naked, noreturn))
void jump_to(uint32_t addr) {
        __asm__ __volatile__ (
                "mov r30, r22\n\t"
                "mov r31, r23\n\t"
                "out %[_RAMPZ], r24\n\t"
                "eijmp\n\t"
                :
                : [_RAMPZ] "I" (_SFR_IO_ADDR(RAMPZ))
        );
}

License

GPL 2 or GPL 3 - choose the one that suits your needs.

Author

Gilhad - 2025