Tilemap Base Address Register: Difference between revisions

Latest revision as of 11:42, 3 November 2024

Number	TBRegisterNumber::$6E
Readable	TBRegisterReadable::Yes
Writable	TBRegisterWritable::Yes
Short Description	ShortDesc::Base address of the 40x32 or 80x32 tile map (similar to text-mode of other computers).

bit 7 = 1 to select bank 7, 0 to select bank 5

bit 6 = Reserved, must be 0

bits 5-0 = MSB of address of the tile map in Bank 5

The value written into bits 5:0 is an offset into 16K bank 5 or 8K bank 7 allowing the tilemap to be placed at any multiple of 256 bytes. The value written here can be thought of as the MSB of an address in bank 5 0x40-0x7f or bank 7 0xc0-0xff. Because bank 7 is only 8K in size, addresses wrap across the 8K boundary instead of the 16K boundary.

The value read back may be treated as a full 8-bit value (with the top two bits equal to zero), i.e. upon writing value $45 the read of this register will produce value $05 (contrary to "reserved" bits in other registers, where read-value should be not assumed by code using it and masking by AND is recommended).

Default value after soft reset corresponds to the address $6C00, i.e. default value is $2C.

@@ Line 5: / Line 5: @@
 |ShortDesc=Base address of the 40x32 or 80x32 tile map (similar to text-mode of other computers).
 }}
-:bits 7-6 = Read back as zero, write values ignored
+:bit 7 = 1 to select bank 7, 0 to select bank 5
-:bits 5-0 = MSB of address of the tilemap in Bank 5
+:bit 6 = Reserved, must be 0
+:bits 5-0 = MSB of address of the tile map in Bank 5
-The value written is an offset into Bank 5 allowing the tilemap to be placed at any multiple of 256 bytes.
+The value written into bits 5:0 is an offset into 16K bank 5 or 8K bank 7 allowing the [[Tilemap|tilemap]]  to be placed at any multiple of 256 bytes. The value written here can be thought of as the MSB of an address in bank 5 0x40-0x7f or bank 7 0xc0-0xff. Because bank 7 is only 8K in size, addresses wrap across the 8K boundary instead of the 16K boundary.
-Writing a physical MSB address as $40-$7F or $C0-$FF range is permitted (the top bits will be ignored).
+The value read back may be treated as a full 8-bit value (with the top two bits equal to zero), i.e. upon writing value $45 the read of this register will produce value $05 (contrary to "reserved" bits in other registers, where read-value should be not assumed by code using it and masking by AND is recommended).
-The value read back may be treated as a full 8-bit value (with the top two bits equal to zero), i.e. upon writing value $45 the read of this register will produce value $05 (contrary to "reserved" bits in other registers, where read-value should be not assumed by code using it and masking by AND is recommended).
+Default value after soft reset corresponds to the address $6C00, i.e. default value is $2C.