(This rant is mirrored from Cadaver's site)
Welcome to my first tutorial written for Covert BitOps about IRQ-Tape Loaders.
There was PC stuff mean while, but that is out of scope.
These loaders are very common in tape originals. They show a bitmap and play music while the game is loading.
Advantages of IRQ based loaders:
Disadventages of IRQ based loaders:
First we should understand how to communicate with the tape recorder. Unlike normal tape recorders, this one uses a digital record which is consisted of pulses. To generate a pulse, all we need to do is to invert bit 3 of the DR register.
Here is a very simple routine to achieve this:
LDA $01 EOR #$08 STA $01
But, how do we read this? The read line is connected to CIA1's FLAG line. A special feature of this line is that it is sensitive to Negative Edge (changing from 1 to 0). Commodore's tape recorder has built-in invertor which inverts this line on every pulse. So we can't read every pulse, but every second pulse. We can read the FLAG line from $DC0D, but we don't have to since we are using an IRQ to achieve this.
You maybe noticed that there is nothing we can use for handshaking. Yeah, it's true, the tape is an asynchronous device. But how do we write 0 or 1 to the tape? Actually, we measure the pause between each pulses. Shorter pause is 0 and longer pause is 1.
From now on, I'll use the term pulse to refer the pause between each succesive pulses.
Here are the pulses used in normal turbo tape:
0 - 216 cycles. 1 - 326 cycles.
When reading, the timer should be somewhere between these two values. The actual value for the timer is 263 cycles.
But, these values are inadequate since we are showing a picture and playing music. Also note the badlines. So we need more tolerant values.
Here are the values I use for an IRQ-Loader:
0 - 446 cycles. 1 - 668 cycles. Timer - 576 cycles.
I mentioned that we can read each second pulse (I don't mean the pause), so when writing to tape we have to divide each pause on two parts. (e.g. 446 cycles = 2 x 223)
Since we have no handshaking lines to use, there has to be another way to stay synchronised. The method is the same as normal turbo tape. We write a series of bytes with value 2 (the kernal routines use another way, which is out of scope). After these bytes, we write a count-down (bytes with values from 9 to 0) used for checking the synchronisation. Then goes the actual data like start address, end address and the program itself. It's important to have both start and end address, so the loader knows how many bytes to load and then read the checksum. In the example below, instead of a count-down, there is a Synchro Check Byte (value = $5A).
It would be fine if we have a way to detect possible errors in transmission. These errors can be caused by wrong azimuth. A way exists and it's very simple. Before loading we initialise a variable to 0 and we add the bytes here. Then we check this with the checksum from the tape. The checksum is always the last byte of the file. An important thing is to know that we can not use OR operation for this as once the value will become $FF it will remain. So we will use EOR (exclusive or) for adding the byte values.
To understand the example below, you need at least a basic knowledge of Assembler and the registers used for communication.
$01 (Data Register - DR) bit 7,6 - undefined bit 5 - Cassette Motor Control (0=on, 1=off) bit 4 - Cassette Switch Sense (1=switch closed) bit 3 - Cassette Data Output Line (invert to write a pulse) bit 2 - CHAREN (0=char rom in) [Not Needed] bit 1 - HIRAM (0=kernal rom out) [Not Needed] bit 0 - LORAM (0=basic rom out) [Not Needed]
$DC0D (CIA1 Inrerrupt Control Register - ICR) bit 7 - interrupt occured (1 if any interrupts) bit 6,5 - unused bit 4 - FLAG bit 3 - SP [Not Needed] bit 2 - Alarm [Not Needed] bit 1 - Timer B bit 0 - Timer A
The best way to understand this stuff is an actual example. Here are the routines used for save and load.
- SAVE SLBS = $AC SHBS = $AD SLBE = $AE SHBE = $AF LLBS = $02 LHBS = $03 LLBE = $04 LHBE = $05 SYNCLEN = $AB CHECKSUM = $D7 BUFFER = $BD BITCNT = $A3 SEI JSR $F838 ; Show 'Press record and play on tape' message and wait LDA #0 ; until the user does that STA $90 ; Initialise system's status variable (0 - no error) LDA # [SLBS] ; LO-Byte of the save add. (This is where the prg. we STA SLBS ; are saving is in memory) LDA # [SHBS] ; HI-Byte... STA SHBS LDA # [SLBE] ; LO-Byte of end address STA SLBE LDA # [SHBE] ; HI-Byte... STA SHBE LDA # [LLBS] ; LO-Byte of the load add. (Where the prg. should be STA LLBS ; loaded) LDA # [LHBS] ; HI-Byte... STA LHBS LDA # [LLBE] ; LO-Byte of end address STA LLBE LDA # [LHBE] ; HI-Byte... STA LHBE LDA $D011 ; Disable VIC-II (This has to be done because of AND #$EF ; badlines STA $D011 LDA $01 ; Disable BASIC ROM and enable cass. motor AND #$DE STA $01 LDX #$00 ; Make a short pause so the cass. motor can reach the LDY #$00 ; needed speed PAUSE DEY BNE PAUSE DEX BNE PAUSE LDA #$02 ; Length of synchronisation record STA SYNCLEN JSR SYNCHRO ; Write synchronisation LDY #$00 LDX #$1E ; X is used for the Time Constant (TC) needed for the CONT LDA $LLBS,Y ; pulse length JSR SAVE LDX #$22 ; TC INY CPY #$04 BNE CONT LDY #$00 STY CHECKSUM LDX #$22 ; TC MORE LDA (SLBS),Y ; Get byte of the program JSR SAVE ; Save the byte on tape LDX #$1F ; TC INC SLBS BNE INCR INC SHBS DEX ; TC = TC - 2 (because INC SHBS steals some cycles) DEX INCR LDA SLBS ; All bytes saved ? CMP SLBE LDA SHBS SBC SHBE BCC MORE ; No, save more LDX #$20 ; TC LDA CHECKSUM ; Save the CHECKSUM JSR SAVE LDA $01 ; Disable cass. motor and enable BASIC ROM again ORA #$21 STA $01 LDA $D011 ; Enable VIC-II ORA #$10 STA $D011 LDA #$01 ; CAS1 (This is a system variable. 1 - cass. motor off) STA $C0 CLI RTS SYNCHRO LDY #$00 LDX #$25 ; TC NEXT LDA #$02 ; 2 is the synchro byte JSR SAVE ; Save it LDX #$23 ; TC INY BNE NEXT ; Save more synchro bytes LDX #$22 ; TC DEC SYNCLEN BNE NEXT ; Save more synchro bytes LDX #$22 ; TC LDA #$5A ; Synchro check byte JSR SAVE ; Save it RTS SAVE STA BUFFER ; Store the byte in a buffer EOR CHECKSUM STA CHECKSUM ; Calculate checksum LDA #$08 STA BITCNT ; Initialise bit counter GO ASL BUFFER JSR WAIT ; Make the needed delay DEC BITCNT BNE GO RTS WAIT LDA #$06 ; Used for inverting the cass. data output line JSR WAIT2 LDX #$2A ; TC LDA #$0E ; Used for inverting the cass. data output line JSR WAIT2 LDX #$26 ; TC RTS WAIT2 DEX BNE WAIT2 ; Make the needed delay BCC B0 ; If recording a 0 bit, no more delay needed LDX #$15 ; TC B1 DEX BNE B1 ; Make more delay if bit 1 B0 STA $01 ; Record a pulse on tape RTS
Some explanations: SLBS, SHBS, SLBE, SHBE must be on zero page and take successive bytes in memory. The same applies for LLBS, LHBS, LLBE, LHBE. There are save and load addresses. You might wonder why. This is because when saving we can have the pic. for example at addr. $6000, but when loading we might need it at addr. (e.g. $E000). TC is the time constant. This is used to make the pause always the same, because different op-codes are processed which take different nr. of cycles.
- LOAD SEI LDA #$00 ; Set black screen STA $D020 STA $D021 LDA #$00 ; Set start and end addresses of the bitmap LDX #$E0 STA $AC STX $AD LDA #$40 LDX #$FF STA $AE STX $AF LDY #$00 ; Clear the bitmap CLRMAP LDA #$00 STA ($AC),Y INC $AC BNE INCR1 INC $AD INCR1 LDA $AC CMP $AE LDA $AD SBC $AF BCC CLRMAP ; If not all cleared, clear more LDA #$00 STA $D011 ; Disable VIC-II (because the bitmap is still not loaded) LDA $01 ; Disable KERNAL and BASIC ROM and enable cass. motor AND #$DD STA $01 LDA #$00 ; Disable VIC-II interupts STA $D01A LDA $D019 ; Acknowledge VIC-II interupts STA $D019 LDA #$7F ; Disable CIAs interupts STA $DC0D STA $DD0D LDA #$90 ; Enable FLAG (cass. input) interrupt STA $DC0D LDA #$40 ; Set timer value to 576 cycles LDX #$02 STA $DC06 STX $DC07 LDA $DC0D ; Acknowledge CIAs interrupts LDA $DD0D LDA #<NMI ; Initialise NMI vector LDX #>NMI STA $FFFA STX $FFFB LDA #<IRQ ; Initialise IRQ vector LDX #>IRQ STA $FFFE STX $FFFF LDA #$08 ; Initialise BIT COUNTER STA BITCNT LDA #$00 ; This flag is used as MUSIC PLAY ON/OFF (0-off) because STA $FE ; music is still not loaded STA $FF ; Flag for the part that is loading CLI NEXT JSR PRGLOAD ; This routine waits until a part is loaded INC $FF ; Increment loading part flag LDA $FF CMP #$01 ; The music has been loaded BNE P1 LDA #$00 ; Initialise music TAX TAY JSR $7000 ; The init add. of the music I'm using LDA #$01 ; Set MUSIC PLAY flag. The music is loaded now, so it STA $FE ; can be played JMP NEXT ; Load next part P1 CMP #$04 ; The BITMAP (screen, color and bitmap) has been loaded BNE P2 LDA #$94 ; Change VIC-II bank ($C000-$FFFF) STA $DD00 LDA #$08 ; Set bitmap memory STA $D018 LDA #$D8 ; Set multi-color mode STA $D016 LDA #$3B ; Set bitmap display mode STA $D011 JMP NEXT ; Load next part P2 CMP #$05 ; First part of game has been loaded (start - $7000) BNE P3 LDA #$00 ; Disable music (the game will overlap it) STA $FE LDX #$18 ; Disable SID voices LDA #$00 CLSID STA $D400,X DEX BPL CLSID JMP NEXT ; Load next part P3 CMP #$06 ; All parts loaded ? BNE NEXT ; No, load next part. SEI LDA $01 ; Disable cass. motor and enable ROMs again ORA #$22 STA $01 LDA #$01 ; CAS1 (This is a system variable. 1 - cass. motor off) STA $C0 LDA #$00 ; SID volume = 0 STA $D418 JSR $E5A8 ; Kernal routine: Initialise VIC-II LDA #$97 ; Change VIC-II bank ($0000-$3FFF) STA $DD00 CLI JMP $0810 ; Jump to program's start address (change this if ; another is used) PRGLOAD LDA #$00 ; Flag: synchronisation reached (0-no, 1-yes) STA $FC STA $FD ; Flag: part loaded (0-no, 1-yes) STA CHECKSUM ; Initialise checksum WAIT LDA $D012 ; A routine for playing the music CMP #$7C BMI NOMUSIC CMP #$84 BPL NOMUSIC LDA $FE ; MUSIC PLAY Flag (1-play, 0-no) BEQ NOMUSIC JSR $7003 ; The play add. of the music I'm using NOMUSIC LDA $FD ; Is the part loaded ? BEQ WAIT RTS IRQ PHA TXA PHA TYA PHA LDA $DC0D ; See if counter has finished counting (no = bit 0, LDX #$19 ; yes = bit 1). Start counter again STX $DC0F LSR A ; Get counter finished flag in CARRY LSR A ROL $BD ; Put the bit in buffer LDA $BD INC $D020 ; Make the screen flash DEC $D020 LDX $FC BEQ GETSYNCHRO ; If no synchronisation, reach it DEC BITCNT LDX BITCNT BEQ CONT JMP RET CONT LDX #$08 STX BITCNT LDX $FC CPX #$01 BEQ TEST ; Synchro Check Byte CPX #$02 BEQ ADDRESS ; Start and end address CPX #$03 BEQ PROGRAM ; Main part CPX #$04 BEQ CHKSUM ; Checksum GETSYNCHRO CMP #$02 ; 2 is the synchro byte BNE RET INC $FC JMP RET TEST CMP #$5A ; $5A is the synchro check byte BEQ GOOD LDA #$00 ; Synchronisation invalid. Retry STA $FC JMP RET GOOD INC $FC LDA #$AC ; $00AC is the place in memory where we keep the start LDX #$00 ; and end address STA $02 STX $03 JMP RET ADDRESS LDY #$00 STA ($02),Y INC $02 LDA $02 CMP #$B0 ; All bytes of the address read ? BNE RET INC $FC JMP RET PROGRAM LDY #$00 STA ($AC),Y ; Store byte in memory EOR CHECKSUM ; Calculate checksum STA CHECKSUM INC $AC BNE INCR2 INC $AD INCR2 LDA $AC CMP $AE LDA $AD SBC $AF BCC RET INC $FC JMP RET CHKSUM CMP CHECKSUM BNE ERROR LDA #$00 ; Set no synchronisation (for the next part) STA $FC LDA #$01 ; Part loaded flag STA $FD JMP RET ERROR LDA $01 ; Disable cass. motor ORA #$20 STA $01 LDA #$0F ; Set lt.grey border indicating load error STA $D020 LDA #$00 ; Set SID volume to 0 STA $D418 LOOP JMP LOOP ; Endless loop RET LDA $DC0D ; Acknowledge interrupt PLA TAY PLA TAX PLA NMI RTI
Some explanations: When playing the music, the program checks to see if the raster reg. is between $7C and $84. We need to have more lines, because of the interrupt. If we check for only one specific raster line the music will play slow as there will be pulses that will exactly happen on this line. If the game exceeds $7000 (because the music starts here) it should be divided in two parts. First part: start address to $7000. Now the loader disables the music. Second part: $7000 to end address. The loader assumes the play button is pressed as the loader itself is saved on the begining of the record and has an autostart. You might need to add a code that will restore the original values of the vectors used for autostart. If the game reaches $C000, you should turn the screen off, when it reaches $7000 (together with the music). This is because the BITMAP is located at $C000.