AltairZ80: Improved choice of boot ROM

AltairZ80/altairz80_defs.h

@@ -52,6 +52,8 @@
 #define LDA_INSTRUCTION         0x3e                        /* op-code for LD A,<8-bit value> instruction   */
 #define UNIT_NO_OFFSET_1        0x37                        /* LD A,<unitno>                                */
 #define UNIT_NO_OFFSET_2        0xb4                        /* LD a,80h | <unitno>                          */
+#define LDB_INSTRUCTION         0x06                        /* op-code for LD B,<8-bit value> instruction   */
+#define START_SECTOR_OFFSET     0x57                        /* LD B,<start_sector_offset>                   */
 
 #define CPU_INDEX_8080          4                           /* index of default PC register */
 

AltairZ80/altairz80_dsk.c

@@ -35,7 +35,6 @@
 
     Address     Mode    Function
     -------     ----    --------
-
         10      Out     Selects and enables Controller and Drive
         10      In      Indicates status of Drive and Controller
         11      Out     Controls Disk Function
@@ -157,6 +156,9 @@
 #define MINI_DISK_SIZE      (MINI_DISK_TRACKS * MINI_DISK_SECT * DSK_SECTSIZE)
 #define MINI_DISK_DELTA     4096                    /* threshold for detecting mini disks           */
 
+#define ALTAIR_DISK_SIZE    337664                  /* size of regular Altair disks                 */
+#define ALTAIR_DISK_DELTA   256                     /* threshold for detecting regular Altair disks */
+
 int32 dsk10(const int32 port, const int32 io, const int32 data);
 int32 dsk11(const int32 port, const int32 io, const int32 data);
 int32 dsk12(const int32 port, const int32 io, const int32 data);
@@ -187,6 +189,7 @@ static int32 sectors_per_track [NUM_OF_DSK] = { DSK_SECT, DSK_SECT, DSK_SECT, DS
                                                 DSK_SECT, DSK_SECT, DSK_SECT, DSK_SECT,
                                                 DSK_SECT, DSK_SECT, DSK_SECT, DSK_SECT,
                                                 DSK_SECT, DSK_SECT, DSK_SECT, DSK_SECT };
+static int32 current_imageSize [NUM_OF_DSK] = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0};
 static int32 tracks         [NUM_OF_DSK]    = { MAX_TRACKS, MAX_TRACKS, MAX_TRACKS, MAX_TRACKS,
                                                 MAX_TRACKS, MAX_TRACKS, MAX_TRACKS, MAX_TRACKS,
                                                 MAX_TRACKS, MAX_TRACKS, MAX_TRACKS, MAX_TRACKS,
@@ -310,6 +313,8 @@ static REG dsk_reg[] = {
                "Number of tracks register array"), REG_CIRC                                 },
     { BRDATAD (SECTPERTRACK, sectors_per_track, 10, 32,  NUM_OF_DSK,
                "Number of sectors per track register array"), REG_CIRC                      },
+    { BRDATAD (IMAGESIZE, current_imageSize, 10, 32,  NUM_OF_DSK,
+               "Size of disk image array"), REG_CIRC + REG_RO                               },
     { DRDATAD (IN9COUNT,     in9_count,         4,
                "Count of IN(9) register"),  REG_RO                                          },
     { DRDATAD (IN9MESSAGE,   in9_message,       4,
@@ -413,6 +418,7 @@ static t_stat dsk_attach(UNIT *uptr, CONST char *cptr) {
      tracks to 16, otherwise, 32 sectors per track. */
 
     imageSize = sim_fsize(uptr -> fileref);
+    current_imageSize[thisUnitIndex] = imageSize;
     sectors_per_track[thisUnitIndex] = (((MINI_DISK_SIZE - MINI_DISK_DELTA < imageSize) &&
                                          (imageSize < MINI_DISK_SIZE + MINI_DISK_DELTA)) ?
                                         MINI_DISK_SECT : DSK_SECT);
@@ -426,7 +432,13 @@ void install_ALTAIRbootROM(void) {
 }
 
 /*  The boot routine modifies the boot ROM in such a way that subsequently
-    the specified disk is used for boot purposes.
+    the specified disk is used for boot purposes. Furthermore, it determines based on the
+    size of the attached disk image which boot ROM should be used. The logic is:
+    - If the size is close to MINI_DISK_SIZE, the ROM for mini disk support is used
+    - If the size is close to ALTAIR_DISK_SIZE, the standard ROM is modified to start
+      loading at track 0, sector 0
+    - Otherwise the standard ROM is is modified to start loading at track 0, sector 8
+    See DSKBOOT.MAC on the cpm2.dsk for the source code of the boot ROM.
 */
 static t_stat dsk_boot(int32 unitno, DEVICE *dptr) {
     if (cpu_unit.flags & (UNIT_CPU_ALTAIRROM | UNIT_CPU_BANKED)) {
@@ -434,14 +446,25 @@ static t_stat dsk_boot(int32 unitno, DEVICE *dptr) {
             const t_bool result = (install_bootrom(alt_bootrom_dsk, BOOTROM_SIZE_DSK,
                                                    ALTAIR_ROM_LOW, TRUE) == SCPE_OK);
             ASSURE(result);
-        } else {
+        } else { /* patch standard ROM with correct unit number and first track */
+
+            /* check whether we are really modifying an LD B,<> instruction */
+            if (bootrom_dsk[START_SECTOR_OFFSET - 1] == LDB_INSTRUCTION)
+                bootrom_dsk[START_SECTOR_OFFSET] =
+                (ALTAIR_DISK_SIZE - ALTAIR_DISK_DELTA < current_imageSize[unitno]) &&
+                (current_imageSize[unitno] < ALTAIR_DISK_SIZE + ALTAIR_DISK_DELTA) ? 0 : 8;
+            else { /* Attempt to modify non LD B,<> instructions is refused. */
+                sim_printf("Incorrect boot ROM first sector offset detected.\n");
+                return SCPE_IERR;
+            }
+
             /* check whether we are really modifying an LD A,<> instruction */
             if ((bootrom_dsk[UNIT_NO_OFFSET_1 - 1] == LDA_INSTRUCTION) &&
                 (bootrom_dsk[UNIT_NO_OFFSET_2 - 1] == LDA_INSTRUCTION)) {
                 bootrom_dsk[UNIT_NO_OFFSET_1] = unitno & 0xff;          /* LD A,<unitno>        */
                 bootrom_dsk[UNIT_NO_OFFSET_2] = 0x80 | (unitno & 0xff); /* LD a,80h | <unitno>  */
             } else { /* Attempt to modify non LD A,<> instructions is refused. */
-                sim_printf("Incorrect boot ROM offsets detected.\n");
+                sim_printf("Incorrect boot ROM unit number offset detected.\n");
                 return SCPE_IERR;
             }
             install_ALTAIRbootROM();                                    /* install modified ROM */