From 493d8b74f6363b3e634f2f0130cc5238ca958af0 Mon Sep 17 00:00:00 2001
From: Matthew Berry <me@mattrb.com>
Date: Sun, 4 Oct 2020 16:10:34 -0700
Subject: [PATCH] remove constant ranges from bus, redesign mem map lookups

Turns out constants in Crystal are slow as _hell_. Thanks Ary for the
help discovering that. Maybe one day Crystal will improve that, but for
now this change over triples performance.
---
 src/crab/bus.cr | 49 +++++++++++++++++++++++++++++++++----------------
 src/crab/ppu.cr |  8 +-------
 2 files changed, 34 insertions(+), 23 deletions(-)

diff --git a/src/crab/bus.cr b/src/crab/bus.cr
index 9ba4944..7a736c6 100644
--- a/src/crab/bus.cr
+++ b/src/crab/bus.cr
@@ -22,14 +22,22 @@ class Bus
 
   def [](index : Int) : Byte
     log "read #{hex_str index.to_u32}"
-    case index
-    when WRAM_BOARD then @wram_board[index - WRAM_BOARD.begin]
-    when WRAM_CHIP  then @wram_chip[index - WRAM_CHIP.begin]
-    when PPU_IO     then @gba.ppu[index]
-    when PPU        then @gba.ppu[index]
-    when CARTRIDGE  then @gba.cartridge[index - CARTRIDGE.begin]
-    when UNUSED     then 0xFF_u8
-    else                 0xFF_u8
+    case bits(index, 24..27)
+    when 0x2 then @wram_board[index & 0x3FFFF]
+    when 0x3 then @wram_chip[index & 0x7FFF]
+    when 0x4
+      io_addr = 0x0FFF_u16 & index
+      if io_addr <= 0x05F
+        @gba.ppu[index]
+      else
+        raise "Unmapped i/o read: #{hex_str index.to_u32}"
+      end
+    when 0x6
+      address = 0x1FFFF_u32 & index
+      address &= ~0x8000 if address > 0x17FFF
+      @gba.ppu.vram[address]
+    when 0x8, 0x9 then @gba.cartridge[index & 0x7FFFFFF]
+    else               raise "Unmapped read: #{hex_str index.to_u32}"
     end
   end
 
@@ -47,14 +55,23 @@ class Bus
 
   def []=(index : Int, value : Byte) : Nil
     log "write #{hex_str index.to_u32} -> #{hex_str value}"
-    case index
-    when WRAM_BOARD then @wram_board[index - WRAM_BOARD.begin] = value
-    when WRAM_CHIP  then @wram_chip[index - WRAM_CHIP.begin] = value
-    when PPU_IO     then @gba.ppu[index] = value
-    when PPU        then @gba.ppu[index] = value
-    when CARTRIDGE  then @gba.cartridge[index - CARTRIDGE.begin] = value # todo is this meant to be writable?
-    when UNUSED     then nil
-    else                 raise "Unimplemented write ~ addr:#{hex_str index.to_u32}, val:#{value}"
+    return if bits(index, 28..31) > 0
+    case bits(index, 24..27)
+    when 0x2 then @wram_board[index & 0x3FFFF] = value
+    when 0x3 then @wram_chip[index & 0x7FFF] = value
+    when 0x4
+      io_addr = 0x0FFF_u16 & index
+      if io_addr <= 0x05F
+        @gba.ppu[index] = value
+      else
+        raise "Unmapped i/o write: #{hex_str index.to_u32}"
+      end
+    when 0x6
+      address = 0x1FFFF_u32 & index
+      address &= ~0x8000 if address > 0x17FFF
+      @gba.ppu.vram[address] = value
+    when 0x8, 0x9 then @gba.cartridge[index & 0x7FFFFFF] = value
+    else               raise "Unmapped write: #{hex_str index.to_u32}"
     end
   end
 
diff --git a/src/crab/ppu.cr b/src/crab/ppu.cr
index 69b7c2f..5c41e7d 100644
--- a/src/crab/ppu.cr
+++ b/src/crab/ppu.cr
@@ -1,8 +1,4 @@
 class PPU
-  BG_OBJ_PALETTE = 0x05000000..0x050003FF
-  VRAM           = 0x06000000..0x06017FFF
-  OAM            = 0x07000000..0x070003FF
-
   # Display timings in cycles
   HDRAW    = 960
   HBLANK   = 272
@@ -29,7 +25,7 @@ class PPU
     num bg_mode, 3                  # (0-5=Video Mode 0-5, 6-7=Prohibited)
   end
 
-  @vram = Bytes.new VRAM.size
+  getter vram = Bytes.new 0x18000
 
   @dispcnt : DISPCNT = DISPCNT.new 0
 
@@ -52,7 +48,6 @@ class PPU
     when 0x04000001 then @dispcnt.value.to_u8!
     when 0x04000002 then 0_u8
     when 0x04000003 then 0_u8
-    when VRAM       then @vram[index = VRAM.begin]
     else                 raise "Unimplemented PPU read ~ addr:#{hex_str index.to_u32}"
     end
   end
@@ -63,7 +58,6 @@ class PPU
     when 0x04000001 then @dispcnt.value = (@dispcnt.value & 0xFF00) | value
     when 0x04000002
     when 0x04000003
-    when VRAM then @vram[index - VRAM.begin] = value
     else           raise "Unimplemented PPU write ~ addr:#{hex_str index.to_u32}, val:#{value}"
     end
   end