Tilemap mode

February 25, 2019 Phoebus Dokos Off Coding, Hardware, Hero Article, News, Resources,

General Description

The tilemap is a hardware character oriented display that comes in two resolutions: 40×32 (320×256 pixels) and 80×32 (640×256 pixels).

The display area on screen is the same as the sprite layer, meaning it overlaps the standard 256×192 area by 32 pixels on all sides. Vertically this is larger than the physical HDMI display, which will cut off the top and bottom character rows making the visible area 40×30 or 80×30, but the full area is visible on VGA.

The obvious application for the tilemap is for a fast, clearly readable and wide multicoloured character display. Less obvious perhaps is that it can also be used to make fast and wide resolution full colour backgrounds with easily animated components.

The tilemap is defined by two data structures.

Data Structures

A. Tilemap

The first data structure is the tilemap itself which indicates what characters occupy each cell on screen. Each tilemap entry is two bytes so for 40×32 resolution, a full size tilemap will occupy 2560 bytes, and for 80×32 resolution the space taken is twice that at 5120 bytes. The tilemap entries are stored in X-major order and each two-byte tilemap entry is stored little endian:

Tilemap Entry

bits 15-12 : palette offset
bit 11 : x mirror
bit 10 : y mirror
bit 9 : rotate
bit 8 : ULA over tilemap (in 512 tile mode, bit 8 of the tile number)
bits 7-0 : tile number

The character displayed is indicated by the “tile number” which can be thought of as an ASCII code. The tile number is normally eight bits allowing up to 256 unique tiles to be displayed but this can be extended to nine bits for 512 unique tiles if 512 tile mode is enabled via the Tilemap Control register.

The other bits are tile attributes that modify how the tile image is drawn. Their function is the same as the equivalent sprite attributes for sprites. Bits apply rotation then mirroring, and colour can be shifted with a palette offset. If 512 tile mode is not enabled, bit 8 will determine if the tile is above or below the ULA display on a per tile basis.

B. Tile Definitions

The second data structure is the tile definitions themselves.

Each tile, identified by tile number, is 8×8 pixels in size with each pixel four bits to select one of 16 colours. A tile definition occupies 32 bytes and is defined in X major order with packing in the X direction in the same way that 4-bit sprites are defined. The 4-bit colour of each pixel is augmented by the 4-bit palette offset from the tilemap in the most significant bits to form an 8-bit colour index that is looked up in the tilemap palette to determine the final 9-bit colour sent to the display.

Tiles are therefore defined using 16 colours with the tilemap palette offset able to act as index into the tilemap palette to vary the display colour. One of the 16 colours is defined as transparent in the Transparency Index register.

Memory Organization & Display Layer

The tilemap is a logical extension of the ULA and its data structures are contained in the ULAís 16k bank 5. If both the ULA and tilemap are enabled, this means that the tilemapís map and tile definitions should be arranged within the 16k to avoid overlap with the display ram used by the ULA.

The tilemap exists on the same display layer as the ULA. The graphics generated by the ULA and tilemap are combined before being forwarded to the SLU layer system as layer U.

Combining ULA & Tilemap

The combination of the ULA and tilemap is done in one of two modes: the standard mode or the stencil mode.

The standard mode uses bit 8 of a tileís tilemap entry to determine if a tile is above or below the ULA. The source of the final pixel generated is then the topmost non-transparent pixel. If the ULA or tilemap is disabled then they are treated as transparent.

The stencil mode will only be applied if both the ULA and tilemap are enabled. In the stencil mode, the final pixel will be transparent if either the ULA or tilemap are transparent. Otherwise the final pixel is a logical AND of the corresponding colour bits. The stencil mode allows one layer to act as a cut-out for the other.

Programming Tilemap mode

(R/W) 0x6B (107) => Tilemap Control
  bit 7    = 1 to enable the tilemap
  bit 6    = 0 for 40x32, 1 for 80x32
  bit 5    = 1 to eliminate the attribute entry in the tilemap
  bit 4    = palette select
  bits 3-2 = Reserved set to 0
  bit 1    = 1 to activate 512 tile mode
  bit 0    = 1 to force tilemap on top of ULA

Bits 7 & 6 enable the tilemap and select resolution. Bit 4 selects one of two tilemap palettes used for final colour lookup. Bit 5 changes the structure of the tilemap so that it contains only 8-bit tilemap entries instead of 16-bit tilemap entries. If 8-bit, the tilemap only contains tile numbers and the attributes are instead taken from nextreg 0x6C.

Bit 1 activates 512 tile mode. In this mode, the “ULA over tilemap” bit in a tile’s attribute is re-purposed as the ninth bit of the tile number, allowing up to 512 unique tiles to be displayed. In this mode, the ULA is always on top of the tilemap.

Bit 0 forces the tilemap to be on top of the ULA. It can be useful in 512 tile mode to change the relative display order of the ULA and tilemap.

(R/W) 0x6C (108) => Default Tilemap Attribute
  bits 7-4 = Palette Offset
  bit 3    = X mirror
  bit 2    = Y mirror
  bit 1    = Rotate
  bit 0    = ULA over tilemap
             (bit 8 of the tile number if 512 tile mode is enabled)

If bit 5 of nextreg 0x6B is set, the tilemapís structure is modified to contain only 8-bit tile numbers instead of the usual 16-bit tilemap entries. In this case, the tile attributes used are taken from this register instead.

(R/W) 0x6E (110) => Tilemap Base Address
  bits 7-6 = Read back as zero, write values ignored
  bits 5-0 = MSB of address of the tilemap in Bank 5

This register determines the tilemapís base address in bank 5. The base address is the MSB of an offset into the 16k bank, allowing the tilemap to begin at any multiple of 256 bytes in the bank. Writing a physical MSB address in 0x40-0x7f or 0xc0-0xff, corresponding to traditional ULA physical addresses, is permitted. The value read back should be treated as a fully significant 8-bit value.

The tilemap will be 40×32 or 80×32 in size depending on the resolution selected in nextreg 0x6B. Each entry in the tilemap is normally two bytes but can be one byte if attributes are eliminated by setting bit 5 of nextreg 0x6B.

(R/W) 0x6F (111) => Tile Definitions Base Address
  bits 7-6 = Read back as zero, write values ignored
  bits 5-0 = MSB of address of tile definitions in Bank 5

This register determines the base address of tile definitions in bank 5. As with nextreg 0x6E, the base address is the MSB of the an offset into the 16k bank, allowing tile definitions to begin at any multiple of 256 bytes in the bank. Writing a physical MSB address in 0x40-0x7f or 0xc0-0xff, corresponding to traditional ULA physical addresses, is permitted. The value read back should be treated as a fully significant 8-bit value.

Each tile definition is 32 bytes in size and is located at address:

Tile Def Base Addr + 32 * (Tile Number)

(R/W) 0x4C (76) => Transparency index for the tilemap
bits 7-4 = Reserved, must be 0
bits 3-0 = Set the index value (0xF after reset)

Defines the transparent colour index for tiles. The 4-bit pixels of a tile definition are compared to this value to determine if they are transparent.

(R/W) 0x43 (67) => Palette Control
  bit 7 = '1' to disable palette write auto-increment.
  bits 6-4 = Select palette for reading or writing:
     000 = ULA first palette
     100 = ULA second palette
     001 = Layer 2 first palette
     101 = Layer 2 second palette
     010 = Sprites first palette 
     110 = Sprites second palette
     011 = Tilemap first palette
     111 = Tilemap second palette
  bit 3 = Select Sprites palette (0 = first palette, 1 = second palette)
  bit 2 = Select Layer 2 palette (0 = first palette, 1 = second palette)
  bit 1 = Select ULA palette (0 = first palette, 1 = second palette)
  bit 0 = Enabe ULANext mode if 1. (0 after a reset)

The tilemap has its own pair of palettes for looking up 9-bit colours. Each tile definition pixel is 4-bits which is combined with the 4-bit palette offset from the tilemap entry in the most significant 8-bits. This 8-bit value is passed through the tilemap palette to generate the final 9-bit colour for the pixel.

(R/W) 0x1B (27) => Clip Window Tilemap
  bits 7-0 = Coord. of the clip window
  1st write = X1 position
  2nd write = X2 position
  3rd write = Y1 position
  4rd write = Y2 position
  The values are 0,159,0,255 after a Reset
  Reads do not advance the clip position

The tilemap display surface extends 32 pixels around the central 256×192 display. The origin of the clip window is the top left corner of this area 32 pixels to the left and 32 pixels above the central 256×192 display. The X coordinates are internally doubled to cover the full 320 pixel width of the surface. The clip window indicates the portion of the tilemap display that is non-transparent and its indicated extent is inclusive; it will extend from X1*2 to X2*2+1 horizontally and from Y1 to Y2 vertically.

(R/W) 0x2F (47) => Tilemap Offset X MSB
bits 7-2 = Reserved, must be 0
bits 1-0 = MSB X Offset
Meaningful Range is 0-319 in 40 char mode, 0-639 in 80 char mode

(R/W) 0x30 (48) => Tilemap Offset X LSB
bits 7-0 = LSB X Offset
Meaningful range is 0-319 in 40 char mode, 0-639 in 80 char mode

(R/W) 0x31 (49) => Tilemap Offset Y
bits 7-0 = Y Offset (0-191)

These are scroll registers for scrolling the tilemap area. As with other layers, the scroll region wraps.

(R/W) 0x68 (104) => ULA Control
  bit 7    = 1 to disable ULA output
  bit 6    = 0 to select the ULA colour for blending in SLU modes 6 & 7
           = 1 to select the ULA/tilemap mix for blending in SLU modes 6 & 7
  bits 5-1 = Reserved must be 0
  bit 0    = 1 to enable stencil mode when both the ULA and tilemap are enabled
            (if either are transparent the result is transparent otherwise the
             result is a logical AND of both colours)

Bit 0 can be set to choose stencil mode for the combined output of the ULA and tilemap. Bit 6 determines what colour is used in SLU modes 6 & 7 where the ULA is combined with Layer 2 to generate highlighting effects.

Changes Since 2.00.26

1. 512 Tile Mode. In 2.00.26, the 512 tile mode was automatically selected when the ULA was disabled. With the ULA disabled, the tilemap attribute bit “ULA on top” was re-purposed to be bit 8 of the tile number. In 2.00.27, selection of the 512 tile mode is moved to bit 1 of Tilemap Control nextreg 0x6B. This way 512 tile mode can be independently chosen without disabling the ULA. The “ULA on top” bit is still taken as bit 8 of the tile number and in the 512 mode, the tilemap is always displayed underneath the ULA.

2. Tilemap Always On Top of ULA. In 2.00.27, bit 0 of Tilemap Control nextreg 0x6B is used to indicate that the tilemap should always be displayed on top of the ULA. This allows the tilemap to display over the ULA when in 512 mode.

Future Direction

The following compatible changes may be applied at a later date:

1. Addition of a bit to Tilemap Control to select a reduced tilemap area of size 32×24 or 64×24 that covers the ULA screen.

2. Addition of a bit to Tilemap Control to select split addressing where the tilemap’s tiles and attributes as well as the tile definitions are split between the two 8k halves of the 16k ULA ram in the same way that the two Timex display files are split. The intention is to make it easier for the tilemap to co-exist with all the display modes of the ULA.

Below you can find two illustrations of Tilemap mode memory usage without and with the ULA disabled kindly provided by Kev Brady.