bios: bank: add support for ROM wraparound emulation

OpenWitch · Jul 28, 2024 · 58547c6 · 58547c6
1 parent f148892
commit 58547c6
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Showing 8 changed files with 90 additions and 77 deletions.
diff --git a/Makefile b/Makefile
@@ -21,8 +21,9 @@ SRC_OS		:= os
 # Defines
 # -------
 
-SRC_BIOS    += bios/bank/$(BIOS_BANK_MAPPER)
 SRC_BIOS    += bios/timer/$(BIOS_TIMER_RTC)
+DEFINES     += -DBIOS_BANK_MAPPER_$(BIOS_BANK_MAPPER)
+DEFINES		+= -DBIOS_BANK_ROM_FORCE_COUNT=$(BIOS_BANK_ROM_FORCE_COUNT)
 
 # Tool paths
 # ----------

diff --git a/bios/bank/2003/bank_get_map.s b/bios/bank/2003/bank_get_map.s
diff --git a/bios/bank/2001/bank_get_map.s → bios/bank/bank_get_map.s b/bios/bank/2001/bank_get_map.s → bios/bank/bank_get_map.s
@@ -25,6 +25,7 @@
 	.intel_syntax noprefix
 
 #include "common.inc"
+#include "bank/bank_macros.inc"
 
 /**
  * INT 18h AH=01h - bank_get_map
@@ -37,10 +38,9 @@
 bank_get_map:
     xor ax, ax
     mov dx, bx
-    xor dh, dh
-    add dx, 0x00C1
-    cmp dx, 0x00C3
-    ja 1f
-    in al, dx
+    bank_map_dx
+    bank_map_read_ax_dx
+    bank_adjust_wraparound_map bl, ax
 1:
     ret
+
diff --git a/bios/bank/2003/bank_set_map.s → bios/bank/bank_macros.inc b/bios/bank/2003/bank_set_map.s → bios/bank/bank_macros.inc
@@ -20,30 +20,56 @@
  * SOFTWARE.
  */
 
-	.arch	i186
-	.code16
-	.intel_syntax noprefix
+// Wrap around ROM indexes to support larger cartridges
+// (bank_set_map variant)
+.macro bank_adjust_wraparound_map reg_bl, reg_cx
+#if BIOS_BANK_ROM_FORCE_COUNT > 0
+    test \reg_bl, \reg_bl
+    jz 1f // SRAM - skip
 
-#include "common.inc"
+    or \reg_cx, (0xFFFF - BIOS_BANK_ROM_FORCE_COUNT)
+1:
+#endif
+.endm
 
-/**
- * INT 18h AH=00h - bank_set_map
- * Input:
- * - BX = Bank
- * - CX = Bank number
- * Output:
- */
-    .global bank_set_map
-bank_set_map:
-    push ax
-    mov ax, cx
-    mov dx, bx
+// Wrap around ROM indexes to support larger cartridges
+// (bank_read variant)
+.macro bank_adjust_wraparound_rw reg_bh, reg_bx
+#if BIOS_BANK_ROM_FORCE_COUNT > 0
+    test \reg_bh, 0x80
+    jz 1f // SRAM - skip
+
+    or \reg_bx, (0xFFFF - BIOS_BANK_ROM_FORCE_COUNT)
+1:
+#endif
+.endm
+
+// bank_get_map/bank_set_map helpers
+#ifdef BIOS_BANK_MAPPER_2003
+.macro bank_map_dx
     xor dh, dh
     add dx, dx
     add dx, 0x00D0
     cmp dx, 0x00D6
     ja 1f
+.endm
+.macro bank_map_read_ax_dx
+    in ax, dx
+.endm
+.macro bank_map_write_ax_dx
     out dx, ax
-1:
-    pop ax
-    ret
+.endm
+#else
+.macro bank_map_dx
+    xor dh, dh
+    add dx, 0x00C1
+    cmp dx, 0x00C3
+    ja 1f
+.endm
+.macro bank_map_read_ax_dx
+    in al, dx
+.endm
+.macro bank_map_write_ax_dx
+    out dx, al
+.endm
+#endif
diff --git a/bios/bank/2001/bank_set_map.s → bios/bank/bank_set_map.s b/bios/bank/2001/bank_set_map.s → bios/bank/bank_set_map.s
@@ -25,6 +25,7 @@
 	.intel_syntax noprefix
 
 #include "common.inc"
+#include "bank/bank_macros.inc"
 
 /**
  * INT 18h AH=00h - bank_set_map
@@ -37,12 +38,10 @@
 bank_set_map:
     push ax
     mov ax, cx
+    bank_adjust_wraparound_map bl, ax
     mov dx, bx
-    xor dh, dh
-    add dx, 0x00C1
-    cmp dx, 0x00C3
-    ja 1f
-    out dx, al
+    bank_map_dx
+    bank_map_write_ax_dx
 1:
     pop ax
     ret
diff --git a/bios/header.s b/bios/header.s
@@ -42,7 +42,34 @@ footer:
 	.byte 1 // Color
 	.byte 0 // Game ID
 	.byte 0x80 // Game version
-	.byte 2 // ROM size
+
+	// ROM size (derived from BIOS_BANK_ROM_FORCE_COUNT)
+#if BIOS_BANK_ROM_FORCE_COUNT == 1024
+	.byte 11
+#elif BIOS_BANK_ROM_FORCE_COUNT == 512
+	.byte 10
+#elif BIOS_BANK_ROM_FORCE_COUNT == 256
+	.byte 9
+#elif BIOS_BANK_ROM_FORCE_COUNT == 128
+	.byte 8
+#elif BIOS_BANK_ROM_FORCE_COUNT == 64
+	.byte 6
+#elif BIOS_BANK_ROM_FORCE_COUNT == 32
+	.byte 4
+#elif BIOS_BANK_ROM_FORCE_COUNT == 16
+	.byte 3
+#elif BIOS_BANK_ROM_FORCE_COUNT == 8
+	.byte 2
+#elif BIOS_BANK_ROM_FORCE_COUNT == 4
+	.byte 1
+#elif BIOS_BANK_ROM_FORCE_COUNT == 1
+	.byte 0
+#elif BIOS_BANK_ROM_FORCE_COUNT == 0
+	.byte 2 // Default - 512 KB
+#else
+# error Unsupported ROM bank count!
+#endif
+
 	.byte 4 // RAM size
 	.byte 4 // Flags
 	.byte 1 // Mapper

diff --git a/config.defaults.mk b/config.defaults.mk
@@ -2,6 +2,7 @@
 # Do not change this file! Change config.mk instead.
 
 BIOS_BANK_MAPPER := 2001
+BIOS_BANK_ROM_FORCE_COUNT := 0
 BIOS_TIMER_RTC := none
 
 NAME := Athena

diff --git a/config.mk b/config.mk
@@ -13,6 +13,11 @@
 
 # BIOS_BANK_MAPPER := 2001
 
+# Force ROM banking to emulate a specified number of banks.
+# For WonderWitch compatibility on >512KB cartridges, set this value to 8.
+
+BIOS_BANK_ROM_FORCE_COUNT := 8
+
 # Select the RTC provided by the cartridge.
 #
 # Available options: