The Hobbit: Routine at 87C9

87C9: Print Proper Character

Used by the routines at 869B and ScrollLine.

Input
A	ASCII value
C	Bit offset
HL	Target screen location

Prints using the custom font.

PrintPropChar

87C9

PUSH AF

Push AF, BC, DE and HL on the stack.

87CA

PUSH BC

87CB

PUSH DE

87CC

PUSH HL

Calculate font graphic address, some examples would be;

Letter	ASCII	* 8	Sum	Font Graphic Address
"!"	$21	$108	($21*$08)+$8722	882A
"A"	$41	$208	($41*$08)+$8722	892A
"a"	$61	$308	($61*$08)+$8722	8A2A

87CD

LD L,A

Store the offset for the font graphic data look-up in HL.

87CE

LD H,$00

Calculate font graphic data address.

87D0

ADD HL,HL

HL=(HL * 8) + $8722

87D1

ADD HL,HL

87D2

ADD HL,HL

87D3

LD DE,$8722

87D6

ADD HL,DE

87D7

EX DE,HL

Store the result in DE, restore the screen location to HL.

87D8

POP HL

87D9

PUSH HL

87DA

LD B,$08

Set a counter for 08 lines.

PrintPropChar_Loop

87DC

LD A,(DE)

A=byte of font graphic data

87DD

PUSH BC

Stash the counter on the stack temporarily.

87DE

DEC C

87DF

INC C

87E0

LD B,$FF

B=$FF as a mask.

87E2

JR Z,PrintPropChar_SkipShift

PrintPropChar_Shift

87E4

SRL A

Shift the font graphic data left.

87E6

SRL B

Shift the mask data left.

87E8

DEC C

Decrease C by one.

87E9

JR NZ,PrintPropChar_Shift

Loop back to PrintPropChar_Shift until we've shifted by the number of bits specified in C.

PrintPropChar_SkipShift

87EB

LD C,A

Stash the font graphic data in C.

87EC

LD A,B

A=invert the shifted mask.

87ED

CPL

87EE

AND (HL)

Merge the mask with the existing data in the screen buffer.

87EF

OR C

OR the font graphic data.

87F0

LD (HL),A

Copy the processed font graphic data to the screen.

87F1

POP BC

Restore the counter off the stack.

87F2

DEC C

87F3

INC C

87F4

JR Z,PrintPropChar_1

87F6

PUSH BC

87F7

LD A,$08

87F9

SUB C

87FA

LD C,A

87FB

LD A,(DE)

87FC

LD B,$FF

PrintPropChar_0

87FE

SLA A

8800

SLA B

8802

DEC C

8803

JR NZ,PrintPropChar_0

8805

LD C,A

8806

LD A,B

8807

CPL

8808

INC HL

8809

AND (HL)

880A

OR C

880B

LD (HL),A

880C

DEC HL

880D

POP BC

PrintPropChar_1

880E

INC DE

880F

INC H

8810

DJNZ PrintPropChar_Loop

Decrease counter by one and loop back to PrintPropChar_Loop until counter is zero.

8812

POP HL

Restore HL, DE and BC off the stack.

8813

POP DE

8814

POP BC

8815

LD A,C

8816

ADD A,$06

8818

CP $08

881A

JR C,PrintPropChar_2

881C

SUB $08

881E

INC L

PrintPropChar_2

881F

LD C,A

8820

POP AF

Restore AF off the stack.

8821

RET

Return.