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Elite on the BBC Micro

Text: printer [Elite-A, I/O processor]

Name: printer [Show more] Type: Subroutine Category: Text Summary: Send the screen to the printer, following a CTRL-P key press
Context: See this subroutine in context in the source code References: This subroutine is called as follows: * RDKEY calls printer

In the following, the escape sequences sent to the printer are standard Epson printer codes.
.printer LDA #2 \ Print ASCII 2 using the VDU routine in the MOS, which JSR print_safe \ means "start sending characters to the printer" LDA #'@' \ Send "ESC @" to the printer to initialise the printer JSR print_esc LDA #'A' \ Send "ESC A 8" to the printer to select line spacing JSR print_esc \ of 8/72 inches (1/9") LDA #8 JSR print_wrch LDA #&60 \ Set SC(1 0) = &6000, so it points to the start of STA SC+1 \ screen memory LDA #0 STA SC .print_view LDA #'K' \ Send "ESC K 0 1" to the printer to select single JSR print_esc \ density graphics (60 dpi) LDA #0 JSR print_wrch LDA #1 JSR print_wrch \ We print the screen one character block at a time \ (where each character block is made up of 8 rows, \ with each row being one byte of screen memory) \ \ We do this in three parts. First, we extract the \ screen memory for the character block and stick it \ into a buffer at print_bits. Second, if this is the \ dashboard, which is in colour, then we process the \ contents of the buffer into pixel patterns (as the \ printer can only print monochrome dots). Finally, we \ send the character block to the printer as a sequence \ of one-pixel-wide vertical slices of eight pixels in \ height, working our way from left to right until the \ character block is printed. And then we move onto the \ next character block until the whole screen is printed .print_outer LDY #7 \ We want to print a single character block of screen \ memory, so set a counter in Y for the 8 rows in the \ character block LDX #&FF \ Set X as an index into the print_bits buffer, starting \ at &FF so the initial INX increments it to an index of \ 0 for the first entry in the buffer .print_copy INX \ Increment the pointer into the print_bits buffer, so \ we can store the character rows in the buffer, \ starting with the bottom row of the character and \ working our way to the top LDA (SC),Y \ Grab the Y-th row from the character block and store STA print_bits,X \ it in the X-th byte of print_bits DEY \ Decrement the character row counter BPL print_copy \ Loop back to print_copy until we have copied all eight \ rows of the character block into the buffer LDA SC+1 \ If the high byte in SC(1 0) < &78 then we are still CMP #&78 \ printing the space view, so jump down to print_inner BCC print_inner \ Otherwise we are printing the dashboard, so we now \ need to process the data in the buffer to use pixel \ patterns, as the structure of each mode 5 screen \ memory byte is interleaved so that the first pixel is \ in bits 0 and 4, the second is in bits 1 and 5, and \ so on (see the deep dive on "Drawing colour pixels in \ mode 5" for more on this) \ \ The idea is that we convert each interleaved pixel \ pair into a two-dot wide pixel, so the colour screen \ gets translated into monochrome pixels that match the \ screen layout \ Note that at this point, X is 7, so we can use it as \ an index into the print_bits buffer as we work our way \ through each byte .print_radar LDY #7 \ For each character pixel row we loop through each bit \ in the interleaved byte, so set a counter in Y for 8 \ bits LDA #0 \ We build up the new pixel row in A, so start with 0 \ so we can fill it up with the correct bit pattern \ The following loop works through each bit in the X-th \ pixel row byte, converting it to a monochrome pattern \ that we can print, which gets stored in the print_bits \ buffer in place of the original colour byte .print_split ASL print_bits,X \ Shift the pixel row byte to the left, so the leftmost \ bit falls off the end and into the C flag BCC print_merge \ If that bit is clear, jump to print_merge to skip the \ following instruction and move onto the next bit ORA print_tone,Y \ The Y-th bit in the original colour byte was set, so \ we grab the Y-th entry from print_tone and OR it into \ the new pixel row byte in A. If you look at the pixel \ rows in the print_tone table, you can see that: \ \ * If Y is 0 or 4, we set monochrome pixels 0 and 1 \ * If Y is 1 or 5, we set monochrome pixels 2 and 3 \ * If Y is 2 or 6, we set monochrome pixels 4 and 5 \ * If Y is 3 or 7, we set monochrome pixels 6 and 7 \ \ The above equates to: \ \ * If colour pixel 0 is non-zero, we set monochrome \ pixels 0 and 1 \ \ * If colour pixel 1 is non-zero, we set monochrome \ pixels 2 and 3 \ \ * If colour pixel 2 is non-zero, we set monochrome \ pixels 4 and 5 \ \ * If colour pixel 3 is non-zero, we set monochrome \ pixels 6 and 7 \ \ So this takes a four-pixel wide character row, and \ creates an eight-pixel wide character row made up of \ two-pixel wide blocks that match the original pattern, \ which is what we want for our printer-friendly version \ of the mode 5 colour dashboard .print_merge DEY \ Decrement the bit counter in Y to move onto the next \ bit in the pixel row byte BPL print_split \ Loop back to print_split until we have shifted all \ eight bits, at which point we have our new monochrome \ pixel row byte STA print_bits,X \ Store the new pixel row byte in A into the X-th entry \ in the print_bits buffer, replacing the unprintable \ colour byte that was there before DEX \ Decrement the pointer into the print_bits buffer BPL print_radar \ Loop back to process the next entry in the print_bits \ buffer until we have processed all eight rows in the \ character block \ We now want to print the character block that we \ stored in the print_bits buffer, which we do by \ printing one-pixel wide vertical slices of the \ character, starting from the left edge of the block \ and working our way to the right edge of the block .print_inner LDY #7 \ We want to work our way through the eight columns in \ the character block, so set a counter in Y for this .print_block LDX #7 \ We want to work our way through the eight rows in \ the character block, so set a counter in X for this, \ starting with the last row as we put the rows into \ the print_bits buffer in reverse order (so this will \ pull them out in the correct order, from top to \ bottom, as we put them into print_bits with the bottom \ row first, so starting with X = 7 will pull out the \ top row first) .print_slice ASL print_bits,X \ Shift the byte for the X-th row in the character block \ to the left, so the leftmost pixel falls off the end \ and into the C flag ROL A \ Shift the pixel from the C flag into bit 0 of A, so we \ build up the vertical slice of 8 pixels in A, one \ pixel at a time DEX \ Decrement the vertical pixel counter BPL print_slice \ Loop back until we have extracted all 8 pixels in the \ vertical slice into A JSR print_wrch \ Send the one-pixel vertical slice to the printer, so \ we print 8 vertical pixels and shift along one pixel DEY \ Decrement the column counter to move onto the next \ vertical slice in the character block BPL print_block \ Loop back until we have printer 8 vertical slices of \ one-pixel width, at which point we have printed the \ whole character block .print_next \ We now want to move onto the next character block on \ the row, so we add 8 to the screen address in SC(1 0) \ as there are 8 bytes in each character block CLC \ Set SC(1 0) = SC(1 0) + 8 LDA SC \ ADC #8 \ starting with the low byte in SC STA SC BNE print_outer \ If the above addition didn't wrap wround back to 0, \ the addition is correct, so loop back up to \ print_outer to print the next character block along \ If we get here then we have just wrapped around to the \ next page in screen memory, which means we have \ reached the end of the current character row and need \ to move onto the next row LDA #13 \ Send a carriage return character (ASCII 13) to the JSR print_wrch \ printer to move the printer head down to the next line INC SC+1 \ Increment the high byte of SC(1 0) to point the screen \ address to the start of the next character row LDX SC+1 \ Set X to the high byte of SC(1 0) plus 1, which points INX \ to the character row after the one we are about to \ print BPL print_view \ If bit 7 of X is clear, this means that X < &80, so \ the high byte of SC(1 0) < &7F, which means we haven't \ yet reached the end of screen memory at &76FF, so \ loop back to print_view to set the graphics density \ again (as we have to do this on each row) and move \ onto the next character row LDA #3 \ Print ASCII 3 using the VDU routine in the MOS, which JMP print_safe \ means "stop sending characters to the printer", and \ return from the subroutine using a tail call \JSR print_safe \ These instructions are commented out in the original \JMP tube_put \ source