Update README.md
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tilleul
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@ -123,7 +123,7 @@ You've just drawn the word "HELLO" the fastest possible way with Applesoft only.
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The problem now is that to `PRINT`/draw all possible combinations of colors you need to be able to `PRINT` all 255 possible values of bytes, including those between 128 to 159, that is the control-characters.
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As print is super-fast compared to `PLOT` (or `HLIN` and `VLIN`), this could be used to display sprites on the lores screen.
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As `PRINT` is super-fast compared to `PLOT` (or `HLIN` and `VLIN`), this could be used to display sprites on the lores screen faster than ever with Applesoft.
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And since what we want to do is use combinations of CTRL-characters to change the text mode **within** strings, we could draw any shape and store the `INVERSE`/`FLASH`/`NORMAL`/`SPECIAL` statements within the string itself !
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@ -132,7 +132,7 @@ The routine will be simple and short, so we can store it in $300. But if you wan
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The first thing we want to do is initialize CSWL/CSWH (and thus activate our routine). To do that we can begin in $300 with a simple init routine that needs to be called at the start of your Applesoft program (or at least before you want to take advantage of the new feature).
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We also want to initialize a flag stating that the "Peculiar mode" is not set yet. The Peculiar mode is that new mode where we allow to print control-characters. I decided that this flag would be stored in zero page $34. This memory location is normally used by the monitor (it is known as `YSAV`) to store temp data but I estimated that there would be no interest in being in the monitor AND want to print control-characters on the lores screen. As this is the only zero page location that I need, I wanted to avoid any of the usual "free" zero pages locations to make sure I don't interfere with possibly other programs that might use these "free" locations.
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We also want to initialize a flag stating that the "Peculiar mode" is not set yet. The "Peculiar mode" is that new mode where we allow to print control-characters. I decided that this flag would be stored in zero page $34. This memory location is normally used by the monitor (it is known as `YSAV`) to store temp data but I estimated that there would be no interest in being in the monitor AND want to print control-characters on the lores screen. As this is the only zero page location that I need, I wanted to avoid any of the usual "free" zero pages locations to make sure I don't interfere with possibly other programs that might use these "free" locations.
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So the code begins with:
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@ -155,8 +155,8 @@ So the first thing we will do is save the values of `A` and `X`. Why these two ?
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We will save `A` on the stack and `X` in a zero page location named `YSAV1` (in `$35`). This location is used by `COUT1` to save the value in `Y` and restore it when leaving. It's not used elsewhere meaning we can use it for the same purpose (except we're going to save `X`) until we actually call `COUT1`.
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Then we are going to verify the value in `A`:
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- if it's CTRL-P then we set the "Peculiar mode" flag on (by writing $FF in it), restore A and X and call `COUT1`
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- if it's CTRL-F then we call the `FLASH` routine in `$F280`, this modifies `A` and `X` registers (X being now $40, the value of the FLASH-mask), we clear `X` and use `X` to reset the "Peculiar mode" flag, we restore `A` and `X` and call `COUT1`
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- if it's CTRL-P then we set the "Peculiar mode" flag on (by writing $FF in it, the important thing being that the hi-bit is set), restore `A` and `X` and call `COUT1`
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- if it's CTRL-F then we call the `FLASH` routine in `$F280`, this modifies `A` and `X` registers (`X` being now $40, the value of the FLASH-mask), we clear `X` and use `X` to reset the "Peculiar mode" flag, we restore `A` and `X` and call `COUT1`
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- if it's CTRL-O then we call the `INVERSE` routine in `$F277`, this again modifies `A` and `X` but `X` is now zero. We use `X` to reset the "Peculiar mode" flag, we restore `A` and `X` and call `COUT1`
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- if it's CTRL-N then we call the `NORMAL` routine in `$F273`. Just like the `INVERSE` routine, this modifies `A` and `X` and `X` is now zero. Again, we use `X` to reset the "Peculiar mode" flag, we restore `A` and `X` and call `COUT1`
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- if it's any other character, then we check the "Peculiar mode" flag. If it's off, we call `COUT1`. If it's on then we need to modify the value of `A` so it fits within $80-$9F. More about that in a moment.
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@ -192,10 +192,9 @@ Here's the code so far:
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337: 4C F0 FD JMP COUT1 ; DISPLAY THE CHARACTER IF POSSIBLE
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33B: A6 34 CHK_P_MODE LDX PECULIAR_MODE ; LOAD PECULIAR MODE FLAG IN X
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33D: E0 FF CPX #$FF ; IS THE FLAG SET ?
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33F: D0 F4 BNE OUT0 ; IF NOT, OUTPUT THE CHARACTER
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33D: 10 F6 BPL OUT0 ; IF HIBIT NOT SET, THEN NOT IN P-MODE, OUTPUT THE CHARACTER
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341: P_MODE
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33F: P_MODE
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```
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Now the only thing left to do is handle the "Peculiar mode". Because this mode is meant to be used with `PRINT` and `PRINT` only (while showing the lores screen), we need to check first if we're busy `PRINT`ing text or if we're doing something else like `LIST` for instance. If the command was `LIST`, we don't want to display weird characters. For example all the space characters (value 32) would be replaced with @ (value zero).
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@ -204,7 +203,7 @@ The way I've found to detect if we're `PRINT`ing something is to check the stack
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So, if our routine was indirectly called from Applesoft, the stack (from the last stacked value) looks like this: `66 DB 4B DB`.
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`$DB66`(+1) is the return address to `OUTDO`, while `$DB4B`(+1) is the return address from the `PRINT` command. The interesting byte in the stack is the 4th. This byte will **never** be `$DB` if our routine comes from anything else but `PRINT`. So, we just need to check the value of the 4th byte. Well, in fact the **5th** byte, because our own routine adds the value of `A` to the stack.
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`$DB66`(+1) is the return address to `OUTDO`, while `$DB4B`(+1) is the return address from the `PRINT` command. The interesting byte in the stack is the 3rd. We can assume this byte will **never** be `$4B` if our routine comes from anything else but `PRINT`. So, we just need to check the value of the 3rd byte. Well, in fact the **4th** byte, because our own routine adds the value of `A` to the stack.
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So if we were not `PRINT`ing, we simply ignore the Peculiar mode and call `COUT1`. But if we're `PRINT`ing we do a bit more work.
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@ -214,26 +213,26 @@ The small setback is that this routine uses `Y`. So far we have not used `Y`. We
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Here goes:
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```Assembly
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341: BA P_MODE TSX ; GET STACK POINTER TO X
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342: BD 05 01 LDA STACK+5,X ; GET MSB OF THE RETURN ADDRESS
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345: C9 DB CMP #$DB ; IS IT FROM PRINT IN $DB49 ?
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347: D0 EC BNE OUT0 ; NO, LET'S CALL COUT1 THEN
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33F: BA P_MODE TSX ; GET STACK POINTER TO X
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340: BD 05 01 LDA STACK+5,X ; GET MSB OF THE RETURN ADDRESS
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343: C9 DB CMP #$DB ; IS IT FROM PRINT IN $DB49 ?
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345: D0 EC BNE OUT0 ; NO, LET'S CALL COUT1 THEN
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349: 68 PLA ; GET CHARACTER VALUE BACK FROM STACK
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34A: C9 A0 CMP #$A0 ; IS IT A CONTROL CHAR (<160)
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34C: 90 E8 BCC OUT1 ; YES, LET'S CALL COUT1 THEN
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347: 68 PLA ; GET CHARACTER VALUE BACK FROM STACK
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348: C9 A0 CMP #$A0 ; IS IT A CONTROL CHAR (<160)
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34A: 90 E8 BCC OUT1 ; YES, LET'S CALL COUT1 THEN
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34E: AA TAX ; SAVE CHAR IN X
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34F: 98 TYA ; SAVE Y IN A
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350: 48 PHA ; STORE VALUE OF Y ON STACK
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351: 8A TXA ; GET BACK CHAR FROM X
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352: 29 9F AND #$9F ; CLEAR BITS 5 & 6, RANGE IS NOW $80-$9F
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354: 20 F0 FB JSR STORADV ; FORCE DISPLAY OF CHAR
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357: 68 PLA ; RESTORE VALUE OF Y FROM STACK
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358: A8 TAY ; BACK TO Y
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359: 8A TXA ; RESTORE A FROM X
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35A: A6 35 LDX YSAV1 ; RESTORE X FROM YSAV1
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35C: 60 RTS ; RETURN TO CALLER
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34C: AA TAX ; SAVE CHAR IN X
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34D: 98 TYA ; SAVE Y IN A
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34E: 48 PHA ; STORE VALUE OF Y ON STACK
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34F: 8A TXA ; GET BACK CHAR FROM X
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350: 29 9F AND #$9F ; CLEAR BITS 5 & 6, RANGE IS NOW $80-$9F
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352: 20 F0 FB JSR STORADV ; FORCE DISPLAY OF CHAR
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355: 68 PLA ; RESTORE VALUE OF Y FROM STACK
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356: A8 TAY ; BACK TO Y
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357: 8A TXA ; RESTORE A FROM X
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358: A6 35 LDX YSAV1 ; RESTORE X FROM YSAV1
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35A: 60 RTS ; RETURN TO CALLER
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```
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And here's the full code:
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@ -245,11 +244,11 @@ And here's the full code:
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320: 00 F0 10 C9 8F D0 05 20
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328: 77 F2 F0 07 C9 8E D0 0B
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330: 20 73 F2 86 34 68 A6 35
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338: 4C F0 FD A6 34 E0 FF D0
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340: F4 BA BD 05 01 C9 DB D0
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348: EC 68 C9 A0 90 E8 AA 98
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350: 48 8A 29 9F 20 F0 FB 68
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358: A8 8A A6 35 60
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338: 4C F0 FD A6 34 10 F6 BA
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340: BD 04 01 C9 4B D0 EE 68
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348: C9 A0 90 EA AA 98 48 8A
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350: 29 9F 20 F0 FB 68 A8 8A
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358: A6 35 60
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```
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## Disabling the OS
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In order to activate the feature, a simple `CALL 768` is enough. But before anything, you must disable DOS (or PRODOS).
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