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Ship hangar: HAL3

[6502 Second Processor version, I/O processor]

Name: HAL3 [Show more] Type: Subroutine Category: Ship hangar Summary: Draw a hangar background line from left to right, stopping when it bumps into existing on-screen content
Context: See this subroutine in context in the source code References: This subroutine is called as follows: * HANGER calls HAL3
.HAL3 TAX \ Store A in X so we can retrieve it after the following \ check and again after updating screen memory AND (SC),Y \ If the pixel we want to draw is non-zero (using A as a BNE HA3 \ mask), then this means it already contains something, \ so we stop drawing because we have run into something \ that's already on-screen, and return from the \ subroutine (as HA3 contains an RTS) TXA \ Retrieve the value of A we stored above, so A now \ contains the pixel mask again AND #RED \ Apply the pixel mask in A to a four-pixel block of \ red pixels, so we now know which bits to set in screen \ memory ORA (SC),Y \ OR the byte with the current contents of screen \ memory, so the pixel we want is set to red (because \ we know the bits are already 0 from the above test) STA (SC),Y \ Store the updated pixel in screen memory TXA \ Retrieve the value of A we stored above, so A now \ contains the pixel mask again LSR A \ Shift A to the right to move on to the next pixel BCC HAL3 \ If bit 0 before the shift was clear (i.e. we didn't \ just do the fourth pixel in this block), loop back to \ HAL3 to check and draw the next pixel TYA \ Set Y = Y + 8 (as we know the C flag is set) to point ADC #7 \ to the next character block along TAY LDA #%10001000 \ Reset the pixel mask in A to the first pixel in the \ new 4-pixel character block BCC HAL3 \ If the above addition didn't overflow, jump back to \ HAL3 to keep drawing the line in the next character \ block RTS \ The addition overflowed, so we have reached the last \ character block in this page of memory, which is the \ end of the line, so we return from the subroutine