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Implement the sector decode function
This was a bear to do, and the product being committed is probably my third (or fourth?) try.
This commit is contained in:
Peter Evans
parent
55c85b0ec6
commit
d2e21b55c4
181
src/apple2.dec.c
181
src/apple2.dec.c
@ -2,118 +2,28 @@
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* apple2.dec.c
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*/
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#include <stdbool.h>
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#include "vm_segment.h"
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static vm_8bit table[] = {
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0x68, 0xa2, 0x5e, 0x5f, 0xff, 0x7f, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
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0x20, 0xa2, 0x5e, 0x5f, 0xff, 0x7f, 0x00, 0x00, 0x49, 0x62, 0x4f, 0x10, 0x01, 0x00, 0x00, 0x00,
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0x40, 0xa2, 0x5e, 0x5f, 0xff, 0x7f, 0x00, 0x00, 0x40, 0xa2, 0x5e, 0x5f, 0xff, 0x7f, 0x00, 0x00,
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0x58, 0xa2, 0x5e, 0x5f, 0xff, 0x7f, 0x00, 0x00, 0x00, 0x50, 0x61, 0x00, 0x01, 0x00, 0x00, 0x00,
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0x00, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x20, 0x55, 0x4f, 0x10, 0x01, 0x00, 0x00, 0x00,
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0xc0, 0x54, 0x4f, 0x10, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
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0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
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0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
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};
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#if 0
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/*
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* This is the "reverse" gcr table from enc.c; given one of the disk
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* bytes, this table can turn them back into a 6-bit byte.
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*/
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static vm_8bit rev62[] = {
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static vm_8bit conv6bit[] = {
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// 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
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0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, // 00
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0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, // 10
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0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, // 20
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0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, // 30
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0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, // 40
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0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, // 50
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0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, // 60
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0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, // 70
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0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, // 80
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0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x02, 0x03, 0x00, 0x04, 0x05, 0x06, // 90
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0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x07, 0x08, 0x00, 0x00, 0x00, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, // a0
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0x00, 0x00, 0x0e, 0x0f, 0x10, 0x11, 0x12, 0x13, 0x00, 0x14, 0x15, 0x16, 0x17, 0x18, 0x19, 0x1a, // b0
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0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x1b, 0x00, 0x1c, 0x1e, 0x1e, // c0
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0x00, 0x00, 0x00, 0x1f, 0x00, 0x00, 0x20, 0x21, 0x00, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27, 0x28, // d0
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0x00, 0x00, 0x00, 0x00, 0x00, 0x29, 0x2a, 0x2b, 0x00, 0x2c, 0x2d, 0x2e, 0x2f, 0x30, 0x31, 0x32, // e0
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0x00, 0x00, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0x00, 0x39, 0x3a, 0x3b, 0x3c, 0x3d, 0x3e, 0x3f, // f0
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0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, // 00
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0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x00, 0x04, 0xff, 0xff, 0x08, 0x0c, 0xff, 0x10, 0x14, 0x18, // 10
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0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x1c, 0x20, 0xff, 0xff, 0xff, 0x24, 0x28, 0x2c, 0x30, 0x34, // 20
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0xff, 0xff, 0x38, 0x3c, 0x40, 0x44, 0x48, 0x4c, 0xff, 0x50, 0x54, 0x58, 0x5c, 0x60, 0x64, 0x68, // 30
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0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x6c, 0xff, 0x70, 0x74, 0x78, // 40
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0xff, 0xff, 0xff, 0x7c, 0xff, 0xff, 0x80, 0x84, 0xff, 0x88, 0x8c, 0x90, 0x94, 0x98, 0x9c, 0xa0, // 50
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0xff, 0xff, 0xff, 0xff, 0xff, 0xa4, 0xa8, 0xac, 0xff, 0xb0, 0xb4, 0xb8, 0xbc, 0xc0, 0xc4, 0xc8, // 60
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0xff, 0xff, 0xcc, 0xd0, 0xd4, 0xd8, 0xdc, 0xe0, 0xff, 0xe4, 0xe8, 0xec, 0xf0, 0xf4, 0xf8, 0xfc, // 70
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};
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#endif
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#if 0
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int
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int
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apple2_dec_sector(vm_segment *dest, vm_segment *src, int doff, int soff)
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{
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int i, orig;
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vm_8bit xoff;
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vm_8bit conv[0x157], xor[0x156];
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vm_8bit conv[0x157];
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vm_8bit xor[0x156];
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vm_8bit lval;
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orig = doff;
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for (i = 0; i < 0x157; i++) {
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conv[i] = table[vm_segment_get(src, soff + i) & 0x7f];
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}
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for (i = 0, lval = 0; i < 0x156; i++) {
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xor[i] = lval ^ conv[i];
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lval = xor[i];
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}
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xoff = 0xac;
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i = 0;
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while (xoff != 0x02) {
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if (xoff >= 0xac) {
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vm_segment_set(dest, doff + xoff,
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(xor[xoff + 0x56] & 0xfc) | ((xor[i] & 0x80) >> 7) | ((xor[i] & 0x40) >> 5));
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}
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xoff -= 0x56;
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vm_segment_set(dest, doff + xoff,
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(xor[xoff + 0x56] & 0xfc) | ((xor[i] & 0x20) >> 5) | ((xor[i] & 0x10) >> 3));
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xoff -= 0x56;
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vm_segment_set(dest, doff + xoff,
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(xor[xoff + 0x56] & 0xfc) | ((xor[i] & 0x08) >> 3) | ((xor[i] & 0x04) >> 1));
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xoff -= 0x53;
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i++;
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}
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#if 0
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for (i = 0; i < 0x56; i++) {
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vm_8bit vac, v56, v00,
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offac, off56;
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offac = i + 0xAC;
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off56 = i + 0x56;
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vac = (xor[offac + 0x56] & 0xfc) | ((xor[i] & 0x80) >> 7) | ((xor[i] & 0x40) >> 5);
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v56 = (xor[off56 + 0x56] & 0xfc) | ((xor[i] & 0x20) >> 5) | ((xor[i] & 0x10) >> 3);
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v00 = (xor[i + 0x56] & 0xfc) | ((xor[i] & 0x08) >> 3) | ((xor[i] & 0x04) >> 1);
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if (i == 0) {
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printf("vac = %02x, v56 = %02x, v00 = %02x\n", vac, v56, v00);
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}
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if (offac > 0xAC) {
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vm_segment_set(dest, doff + offac, vac);
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}
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vm_segment_set(dest, doff + off56, v56);
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vm_segment_set(dest, doff + i, v00);
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}
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#endif
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return 256;
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}
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#endif
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int
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apple2_dec_sector(vm_segment *dest, vm_segment *src, int doff, int soff)
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{
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vm_8bit data[500];
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int i;
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int prologue = soff;
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@ -131,12 +41,63 @@ apple2_dec_sector(vm_segment *dest, vm_segment *src, int doff, int soff)
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return 0;
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}
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// We want to start with just the data field from the encoded
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// sector; this means stripping out any metadata and self-sync
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// bytes.
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for (i = soff + 9; i < epilogue; i++) {
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data[i - soff - 9] = vm_segment_get(src, i);
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// Here we mean to convert the 6-and-2 encoded bytes back into its
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// first intermediate form
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for (i = 0; i < 0x157; i++) {
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conv[i] = conv6bit[vm_segment_get(src, soff + i + 9) & 0x7f];
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}
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// Originally, we XOR'd each byte when encoding; so we need to do
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// another XOR, in pretty much the same manner.
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for (i = 0, lval = 0; i < 0x156; i++) {
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xor[i] = lval ^ conv[i];
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lval = xor[i];
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}
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// Now we need to copy every byte back into its form that would be
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// found on the original disk image. We're using the same sort of
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// loop that jumps around three different sections of the array per
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// iteration.
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for (i = 0; i < 0x56; i++) {
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vm_8bit offac, off56;
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vm_8bit vac, v56, v00;
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offac = i + 0xac;
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off56 = i + 0x56;
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// Recall that the least significant bits are packed into the
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// first 86 (0x56) bytes of the 6-and-2 scheme block. So what
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// we're doing here is grabbing the 6 _most_ significant bits
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// (which is offac + 0x56), then using an OR to pack on the
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// least significant bits from the those first 86 bytes.
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vac =
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(xor[offac + 0x56] & 0xfc) |
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((xor[i] & 0x80) >> 7) |
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((xor[i] & 0x40) >> 5);
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v56 =
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(xor[off56 + 0x56] & 0xfc) |
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((xor[i] & 0x20) >> 5) |
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((xor[i] & 0x10) >> 3);
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v00 =
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(xor[i + 0x56] & 0xfc) |
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((xor[i] & 0x08) >> 3) |
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((xor[i] & 0x04) >> 1);
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// If we wrap around to 00 or 01, as will likely do with offac,
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// don't do the set (it gets set with doff+i and v00).
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if (offac >= 0xac) {
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vm_segment_set(dest, doff + offac, vac);
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}
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// Set the rest!
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vm_segment_set(dest, doff + off56, v56);
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vm_segment_set(dest, doff + i, v00);
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}
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// Finally, we always return 256 since that's all we will be able to
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// write from the given block (the validation of which is done by
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// checking prologue/epilogue bytes first in this function).
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return 256;
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}
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