Routines
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56309: Draw/erase graphics - calculate screen display address position
Used by the routine at 56216.
Input
B Number of horizontal pixel shifts/rotates (rotated at 56363) needed
C Pixel number in character row (0-7)
D Vertical pixel row at which to draw the graphic, from the top of the screen display
E Horizontal pixel column at which to draw the graphic, from the left of the screen display
This routine calculates the screen display address position to draw the graphic at, and checks if it's encroaching on the left or right border of the screen
56309 LD A,1 This is the flag indicating whether (any part of the graphic) is visible on-screen (0 = not visible, 1 = visible)
56311 LD (60117),A It's set here to visible (1) as part of the item is visible.
This calculates the LOW BYTE of the screen display address
56314 LD A,E Horizontal pixel position, from left of screen display
56315 RRA Divide by 8 to get the screen (character) column
56316 RRA
56317 RRA
56318 AND 31 Filter out unnecessary bits (5-7) that may have been affected by the RRA instructions, just keeping bits 0-4 (values 0-31)
56320 LD E,A Store in E register
56321 LD A,D Get the calculated vertical (Y) pixel position - these need to go into bits 5-7 of the byte currently stored in E
56322 RLA Character row is currently indicated by bits 3-5 so shift them left twice to bits 5-7
56323 RLA
56324 AND 224 Get those top three bits (filter out any others)...
56326 ADD A,E ...and combine them with bits 0-4 which hold the column number, to form the screen display address low byte
56327 LD E,A Store back in E register
This routine calculates the HIGH BYTE of the screen address
56328 LD A,D Vertical pixel position, from top of screen display
56329 RRA Divide by 8 to get screen (character) row
56330 RRA
56331 RRA
56332 AND 24 Bits 3 (8) and 4 (16) include the screen display address high byte character row (0-24)
56334 ADD A,64 Set bit 6 (always set for screen addresses)
56336 ADD A,C C contains the pixel row number in the character line (0-7). Add this to the high byte of the address
56337 LD D,A Put the calculated screen address high byte into the D register
56338 LD A,(60113) Number of bytes in the graphic tile to draw
56341 LD C,A Store in C as a counter (checked later at 56432)
Check if we're in the decorative screen border. Tiles are drawn (character) column by column, so we only need to check for the inner border (columns 1 & 30), not the outer ones (columns 0 & 31).
Mirrored graphic tiles have 'reversed' offsets so they tend to be drawn right-to-left, so we need to check both left and right columns.
56342 LD A,E Retrieve the screen display low byte from the E register
56343 AND 31 Bits 0-4 (values 0-31) give us the screen vertical (character) column
56345 CP 1 Check if we're into the left border (column 1)
56347 JP Z,56533
56350 CP 29 Check if we're into the right border (column 29)
56352 JP Z,56457
Start of tile graphic draw/erase loop - jump back here from 56435
Shifts graphic so that shifted graphics byte contained in 2 bytes, H & L, ready to draw on screen
56355 LD A,(HL) Get graphics byte
56356 INC HL ...And move graphic address pointer along, ready for next one
56357 CP 0 Byte empty?
56359 JR Z,56406 ...If so, skip this byte, don't bother shifting or drawing
56361 PUSH HL Store address pointer
Shifts graphic so that the right-shifted graphics byte is contained in 2 bytes, H and L, ready to draw on screen
56362 LD H,A H contains the unshifted graphic byte
56363 LD L,0 Clear L register ready to receive shifted bits
56365 LD A,B Temp store for the 'shift counter' (containing number of right-shifts needed)
56366 CP 0 Any more shifts needed?
56368 JR Z,56377 ...If not, skip out of the shift loop
56370 RR H Rotate byte in H register into L register
56372 RR L
56374 DJNZ 56370 Repeat for number of shifts (counter)
56376 LD B,A Reset the shift counter (from A register)
56377 EX DE,HL Swap registers so that D & E contain the byte values to draw, and HL contains the screen display address to draw at
56378 LD A,(60111) Check draw/erase flag
56381 CP 1 0 = Erase, 1 = Draw
56383 JR Z,56396 If flag = 1, jump to DRAW routine. Otherwise ERASE (next routine)
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