MSP430 20-bit read/write from 16-bit-only compiler

highmem.c

/* Public domain software
 * MSP430 20-bit data copy runnable from within a 16-bit-only compiler (e.g. mspgcc 4.6.3)
 */
#include <msp430.h>
#include <stdint.h>

void highmem_write(uint32_t dstaddr, void *srcaddr, uint16_t len)
{
        // Copy <len> bytes in units of 2-bytes per transfer from srcaddr[] to dstaddr[]
        len &= ~1;
        if (!len)
                return;
        asm volatile(
                "adda #65536, %B[dstaddr]               \n\t"  // Upper 16 bits need shifting
                "adda %B[dstaddr], %A[dstaddr]          \n\t"  // Combine with lower 16 bits
                "                                       \n\t"
                "movx.w @%[srcptr]+, @%A[dstaddr]       \n\t"  // Copy 2 bytes at a time, incrementing srcptr+=2
                "incda %A[dstaddr]                      \n\t"  // Increment dstptr in step +=2
                "decdx.w %[len]                         \n\t"
                "jz .+4                                 \n\t"
                "jmp .-14                               \n\t"
                : [dstaddr] "+r" (dstaddr), [srcptr] "+r" (srcaddr), [len] "+r" (len)
                :
                :       );
}

void highmem_read(uint32_t srcaddr, void *dstaddr, uint16_t len)
{
        // Copy <len> bytes in units of 2-bytes per transfer from srcaddr[] to dstaddr[]
        len &= ~1;
        if (!len)
                return;
        asm volatile(
                "adda #65536, %B[srcaddr]               \n\t"  // Upper 16 bits need shifting
                "adda %B[srcaddr], %A[srcaddr]          \n\t"  // Combine with lower 16 bits
                "                                       \n\t"
                "movx.w @%A[srcaddr]+, @%[dstptr]       \n\t"  // Copy 2 bytes at a time, incrementing srcptr+=2
                "incda %[dstptr]                        \n\t"  // Increment dstptr in step +=2
                "decdx.w %[len]                         \n\t"
                "jz .+4                                 \n\t"
                "jmp .-14                               \n\t"
                : [dstptr] "+r" (dstaddr), [srcaddr] "+r" (srcaddr), [len] "+r" (len)
                :
                :       );
}