Wheelie: Routine at 6700

6700: Fill New Attribute Column

Fills the edge column which has been newly placed on the screen.

FillNewAttributeColumn 6700 LD DE,$7802 DE=7802.

6703 LD B,$10 Set a counter in B to process 10 rows.

FillNewAttributeColumn_Loop 6705 LD A,(DE) Read the terrain data into A.

6706 INC E Move to the next column of terrain data.

6707 LD C,$04 Set the default colour in C to GREEN.

6709 CP $0F Jump to SetNewAttributeColumn_Colour if the terrain byte is lower than 0F or higher than 90.

670B JR C,SetNewAttributeColumn_Colour

670D CP $90

670F JR NC,SetNewAttributeColumn_Colour

6711 INC C Increment the colour byte in C by one (to CYAN).

6712 CP $3D Jump to SetNewAttributeColumn_Colour if the terrain byte is lower than 3D.

6714 JR C,SetNewAttributeColumn_Colour

6716 INC C Increment the colour byte in C by one (to YELLOW).

6717 CP $51 Jump to SetNewAttributeColumn_Colour if #the terrain byte is lower than 51.

6719 JR C,SetNewAttributeColumn_Colour

671B LD C,$02 Set the colour in C to RED.

671D CP $6F Jump to SetNewAttributeColumn_Colour if the terrain byte is lower than 6F.

671F JR C,SetNewAttributeColumn_Colour

6721 INC C Increment the colour byte in C by one (to MAGENTA).

SetNewAttributeColumn_Colour 6722 LD (HL),C Write the attribute byte in C to the attribute buffer pointer.

Move down to the next row in the same column.

This is quite clever, and very subtle. It's probably easier here to show examples:

Input in HL	+20 Bytes
581F	583F
583F	585F
58DF	58FF
58FF	591F

From a quick glance, the following code looks pretty straight-forward, just a little strange: L = L + 20 then H = H + carry - L.

The cleverness here is with the carry flag and the ZX Spectrum attribute buffer layout.

When adding 20 to L any overflow past FF causes the carry flag to be set and this moves into the next page of attribute memory. The SUB L instruction then removes the low byte from the high byte calculation, leaving just the original high byte + the carry flag.

6723 LD A,$20 L+=20 to move down one position.

6725 ADD A,L

6726 LD L,A

6727 ADC A,H Add the carry to H.

6728 SUB L

6729 LD H,A

672A DJNZ FillNewAttributeColumn_Loop Decrease the row counter by one and loop back to FillNewAttributeColumn_Loop until all the rows have been processed.

672C RET Return.