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Elite on the BBC Micro

Ship hanger: HA2 [Elite-A, I/O processor]

Name: HA2 [Show more] Type: Subroutine Category: Ship hanger Summary: Implement the picture_v command (draw vertical lines for the ship hanger background)
Context: See this subroutine in context in the source code References: This subroutine is called as follows: * tube_table calls HA2

This routine is run when the parasite sends a picture_v command. It draws the specified number of vertical lines for the ship hanger's background.
.HA2 JSR tube_get \ Get the parameter from the parasite for the command: \ \ picture_v(line_count) \ \ and store it as follows: \ \ * A = the number of vertical lines to draw AND #%11111000 \ Each character block contains 8 pixel rows, so to get \ the address of the first byte in the character block \ that we need to draw into, as an offset from the start \ of the row, we clear bits 0-2 STA SC \ Set the low byte of SC(1 0) to this value LDX #&60 \ Set the high byte of SC(1 0) to &60, the high byte of STX SC+1 \ the start of screen, so SC(1 0) now points to the \ address where the line starts LDX #%10000000 \ Set a mask in X to the first pixel the 8-pixel byte LDY #1 \ We are going to start drawing the line from the second \ pixel from the top (to avoid drawing on the 1-pixel \ border), so set Y to 1 to point to the second row in \ the first character block .HAL7 TXA \ Copy the pixel mask to A AND (SC),Y \ If the pixel we want to draw is non-zero (using A as a BNE HA6 \ mask), then this means it already contains something, \ so jump to HA6 to stop drawing this line TXA \ Copy the pixel mask to A again ORA (SC),Y \ OR the byte with the current contents of screen \ memory, so the pixel we want is set STA (SC),Y \ Store the updated pixel in screen memory INY \ Increment Y to point to the next row in the character \ block, i.e. the next pixel down CPY #8 \ Loop back to HAL7 to draw this next pixel until we BNE HAL7 \ have drawn all 8 in the character block INC SC+1 \ Point SC(1 0) to the next page in memory, i.e. the \ next character row LDY #0 \ Set Y = 0 to point to the first row in this character \ block BEQ HAL7 \ Loop back up to HAL7 to keep drawing the line (this \ BEQ is effectively a JMP as Y is always zero) .HA6 RTS \ Return from the subroutine