Avalon: Routine at E2FA

E2FA: Character/text printing

Used by the routine at E2D0.

Input
A	Offset for letter to print, e.g. 0 = A, 1 = B (see 6D24)

This routine is used at E0C2 to print a single text character. It allows for text to be printed at any pixel line (rather than starting from the top of an 8x8 pixel square).

The character set text graphics are stored at 6D24.

E2FA

LD HL,$6D24

Start address of character set ('A')

E2FD

LD D,$00

Load DE with offset of character (0 = A, 1 = B, 2 = C etc.)

E2FF

LD E,A

Outer loop counter

E300

LD B,$08

8 bytes in each character

E302

ADD HL,DE

Add offset to base address

E303

DJNZ $E302

...and repeat 8 times (x8) to find the address of the character to print

E305

EX DE,HL

Character address now stored in DE

E306

LD HL,($EAA4)

...Get screen display address (???) to print text at

E309

LD C,H

Store high byte in C for retrieval later

E30A

LD B,$08

8 pixel row bytes to print

E30C

LD A,(DE)

Get first byte of letter graphic

E30D

INC DE

Move to the next byte for later

E30E

LD (HL),A

Print byte onto screen

E30F

LD A,H

Get the high byte of the screen address

E310

AND $07

Look at the pixel row of the line (0-7)

E312

INC A

Increment and check if it's gone over to the next character row

E313

CP $08

E315

JR NZ,$E321

E317

LD A,H

Get the high byte of the display address

E318

AND $F8

Sets the pixel line in the character row to 0 as we're into a new character row

E31A

LD H,A

...and restore back to H (high byte of display address)

E31B

LD A,L

Move the low byte of the display address to the next character row down, and store

E31C

ADD A,$20

E31E

LD L,A

E31F

JR $E322

INC H in next line of code can now be skipped as HL contains the correct display address position

Jump in from E315, where the pixel row is still 0-7 (same character row) so can just increment the high byte of display address (H register) to get to the right place:

E321

INC H

E322

DJNZ $E30C

Repeat printing pixel rows until all 8 bytes of the character are printed on screen

E324

LD A,L

Restore L (the low byte of the screen address) to its starting column position + 1, i.e. 1 character to the right, ready to print the next letter

E325

SUB $1F

E327

LD L,A

E328

LD H,C

..and also restore H (the high byte) to its starting position

E329

LD ($EAA4),HL

...and store display address position ready to print next character.

Letter/character has been printed

E32C

LD A,($EAA3)

Decrement the 'number-of-letters-to-print' counter (columns) (I think)

E32F

DEC A

E330

LD ($EAA3),A

E333

JR NZ,$E2E0

Continue printing until finished

E335

RET