32 columns by 24 rows, implemented using a pixel resolution of 256 by 192 pixels
Only certain register combos are allowed. Pages 44/45 of the 'mastering machine code' book describe the various combos of loading into a register, or two registers from a constant or memory location.
Registers like D, E (and the combined DE) are special. HL is too. A is special as well. You can't just use them in any combo it seems.
Start End Length Description #FF58 #FFFF 168 Reserved #5CCB #FF57 41,612 Free memory #5CC0 #5CCA 11 Reserved #5C00 #5CBF 192 System variables #5B00 #5BFF 256 Printer buffer #5800 #5AFF 768 Attributes #4000 #57FF 6,144 Pixel data #0000 #3FFF 16,384 Basic ROM
Addressing attributes is as easy as you would expect, starting at #5800 there are 32 attributes per screen row and 24 rows. 0 1 2 3 4 5 6 7 8 9 A B C D E F … 1F #5800 #5820 #5840 X #5860 … #5EAO
To set the attributes for character (3, 2) assuming (0,0) is at the top left of the screen you write to address #5800 + ((2*32)+ 3) or #5843 (and X marks the spot).
Each block of 8x8 pixels has a single byte of attribute data packed as follows 7 6 5 4 3 2 1 0 F B P2 P2 P1 I0 I1 I0
Bit 7 if set indicates the colour flashes between the fore and back colours.
Bit 6 if set indicates the colours are rendered bright.
Bits 5 to 3 contain the paper (background) colour 0..7
Bits 2 to 0 contain the ink (foreground) colour 0..7
The flash attribute alternates a cell between its foreground and background colours on a timer, it is of limited use in games.
The bright attribute makes the foreground and background colours, err…, brighter. The bright attribute nearly doubles the effective number of colours the spectrum could display. I say nearly because bright black is still black.
The colour value 0..7 is the index into the colour table, the colour values at those indexes and their bright equivalents are*: Decimal Binary Colour Normal Bright 0 000 Black 1 001 Blue 2 010 Red 3 011 Magenta 4 100 Green 5 101 Cyan 6 110 Yellow 7 111 White
screen_width_pixels: .equ 256 screen_height_pixels: .equ 192 screen_width_chars: .equ 32 screen_height_chars: .equ 24
screen_start: .equ #4000 screen_size: .equ screen_width_chars * screen_height_pixels attr_start: .equ #5800 attributes_length: .equ screen_width_chars * screen_height_chars
black: .equ %000000 blue: .equ %000001 red: .equ %000010 magenta: .equ %000011 green: .equ %000100 cyan: .equ %000101 yellow: .equ %000110 white: .equ %000111
pBlack: .equ black << 3 pBlue: .equ blue << 3 pRed: .equ red << 3 pMagenta: .equ magenta << 3 pGreen: .equ green << 3 pCyan: .equ cyan << 3 pYellow: .equ yellow << 3 pWhite: .equ white << 3
bright: .equ %1000000
The cls_attributes sub-routine is used to set all attributes for the screen to the same value
;
; IN - A contains the attribute value to initialize the screen to
; OUT - Trashes HL, DE, BC
;
cls_attributes:
ld hl, attr_start ; start at attribute start
ld de, attr_start + 1 ; copy to next address in attributes
ld bc, attributes_length - 1 ; 'loop' attribute size minus 1 times
ld (hl), a ; initialize the first attribute
ldir ; fill the attributes
ret
Using the cls_attributes method is as simple as:
ld a, pBlue | yellow | bright
call cls_attributes
Pixel location
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 0 1 0 Y7 Y6 Y2 Y1 Y0 Y5 Y4 Y3 X4 X3 X2 X1 X0
ld a,b ; Work on the upper byte of the address and %00000111 ; a = Y2 Y1 y0 or %01000000 ; first three bits are always 010 ld h,a ; store in h ld a,b ; get bits Y7, Y6 rra ; move them into place rra ; rra ; and %00011000 ; mask off or h ; a = 0 1 0 Y7 Y6 Y2 Y1 Y0 ld h,a ; calculation of h is now complete ld a,b ; get y rla ; rla ; and %11100000 ; a = y5 y4 y3 0 0 0 0 0 ld l,a ; store in l ld a,c ; and %00011111 ; a = X4 X3 X2 X1 or l ; a = Y5 Y4 Y3 X4 X3 X2 X1 ld l,a ; calculation of l is complete ret
The code to perform our address translation (remember B is the Y coordinate and C the character X) becomes:
ld h, 0 ld l, b ; hl = Y add hl, hl ; hl = Y * 2 ld de, screen_map ; de = screen_map add hl, de ; hl = screen_map + (row * 2) ld a, (hl) ; implements ld hl, (hl) inc hl ld h, (hl) ld l, a ; hl = address of first pixel from screen_map ld d, 0 ld e, c ; de = X add hl, de ; add the char X offset ret ; return screen_map[Y*2] + X
screen_map: .defw #4000, #4100, #4200, #4300, #4400, #4500, #4600, #4700, #4020, #4120, #4220, #4320