480.c

#include <stdio.h>
#include <dos.h>

typedef unsigned char uint8_t;

void check_b(uint8_t far *p) {
   uint8_t o = *p;
   *p = ~o;
   if (*p == o) {
      printf(" [%02x]", o);
   } else {
      *p = o;
      printf(" %02x", o);
   }
}

void check(uint8_t far *p) {
   int i;
   printf("%04x:%04x:", FP_SEG(p), FP_OFF(p));
   for (i = 0; i < 8; ++i) {
      check_b(p + i);
   }
   printf("\n");
}

int main(int argc, char *argv[]) {
   int i, j;
   unsigned char far *d0, far *d8, far *e0, far *e8, far *f8;
   unsigned char o480, o404;

   d0 = MK_FP(0xd000, 0);
   d8 = MK_FP(0xd800, 0);
   e0 = MK_FP(0xe000, 0);
   e8 = MK_FP(0xe800, 0);
   f8 = MK_FP(0xf800, 0);

   o404 = inp(0x404);
   printf("Original 404h: %02x\n", o404);
   o480 = inp(0x480);
   printf("Original 480h: %02x\n", o480);

   for (j = 0; j < 2; ++j) {
      outp(0x404, j << 7);
      for (i = 0; i < 4; ++i) {
         outp(0x480, i);
         printf("\n[404h = %02x, 480h = %02x]\n", j << 7, i);

         check(d0);
         check(d8);
         check(e0);
         check(e8);
         check(f8);
      }
   }

   outp(0x480, o480);
   outp(0x404, o404);
   return 0;
}

out.txt

Original 404h: 00
Original 480h: 00

[404h = 00, 480h = 00]
d000:0000: [ff] [ff] [ff] [ff] [ff] [ff] [ff] [ff]
d800:0000: [ff] [ff] [ff] [ff] [ff] [ff] [ff] [ff]
e000:0000: [ff] [ff] [ff] [ff] [ff] [ff] [ff] [ff]
e800:0000: [ff] [ff] [ff] [ff] [ff] [ff] [ff] [ff]
f800:0000: [07] [01] [73] [00] [00] [00] [00] [00]

[404h = 00, 480h = 01]
d000:0000: [00] [00] [00] [01] [00] [00] [20] [06]
d800:0000: 01 ff 42 4f 4f 54 a8 00
e000:0000: [ff] [ff] [ff] [ff] [ff] [ff] [ff] [ff]
e800:0000: [ff] [ff] [ff] [ff] [ff] [ff] [ff] [ff]
f800:0000: [07] [01] [73] [00] [00] [00] [00] [00]

[404h = 00, 480h = 02]
d000:0000: [ff] [ff] [ff] [ff] [ff] [ff] [ff] [ff]
d800:0000: [ff] [ff] [ff] [ff] [ff] [ff] [ff] [ff]
e000:0000: [ff] [ff] [ff] [ff] [ff] [ff] [ff] [ff]
e800:0000: [ff] [ff] [ff] [ff] [ff] [ff] [ff] [ff]
f800:0000: 00 00 00 00 00 00 00 00

[404h = 00, 480h = 03]
d000:0000: [00] [00] [00] [01] [00] [00] [20] [06]
d800:0000: 01 ff 42 4f 4f 54 a8 00
e000:0000: [ff] [ff] [ff] [ff] [ff] [ff] [ff] [ff]
e800:0000: [ff] [ff] [ff] [ff] [ff] [ff] [ff] [ff]
f800:0000: 00 00 00 00 00 00 00 00

[404h = 80, 480h = 00]
d000:0000: 00 00 00 00 00 00 00 00
d800:0000: 00 00 00 00 00 00 00 00
e000:0000: 00 00 00 00 00 00 00 00
e800:0000: 00 00 00 00 00 00 00 00
f800:0000: [07] [01] [73] [00] [00] [00] [00] [00]

[404h = 80, 480h = 01]
d000:0000: 00 00 00 00 00 00 00 00
d800:0000: 00 00 00 00 00 00 00 00
e000:0000: 00 00 00 00 00 00 00 00
e800:0000: 00 00 00 00 00 00 00 00
f800:0000: [07] [01] [73] [00] [00] [00] [00] [00]

[404h = 80, 480h = 02]
d000:0000: 00 00 00 00 00 00 00 00
d800:0000: 00 00 00 00 00 00 00 00
e000:0000: 00 00 00 00 00 00 00 00
e800:0000: 00 00 00 00 00 00 00 00
f800:0000: 00 00 00 00 00 00 00 00

[404h = 80, 480h = 03]
d000:0000: 00 00 00 00 00 00 00 00
d800:0000: 00 00 00 00 00 00 00 00
e000:0000: 00 00 00 00 00 00 00 00
e800:0000: 00 00 00 00 00 00 00 00
f800:0000: 00 00 00 00 00 00 00 00