The threshold value for some Commodore 64 loaders

@DrWatson

Published in 

Commodore64

 · 16 May 2021

I'm referring to those loaders using an IRQ handler routine and FLAG line(1) interrupt. For some reason, the Threshold value for them is often omitted in docs.

Here there's a note about how to extract it. I will refer to the ASM code of a loader I just found, which uses CIA #1, Timer B. Other loaders may use a different combination, but the results are the same.

Before changing the vector to main IRQ handler (by writing to $FFFE/$FFFF) we have:

 LDA #$1F       ; disable Timer A interrupt 
 STA $DC0D      ; disable Timer B interrupt 
                ; disable TOD clock alarm interrupt 
                ; disable serial shift register interrupt 
                ; disable FLAG line(1) interrupt 
 ... 
 LDA #$A0       ; Timer B Countdown start value 
 STA $DC06      
 LDA #$03        
 STA $DC07     

The loader's own IRQ handler looks this way:

 PHA 
 TYA 
 PHA 
 LDA $DC07      

 LDY #$11       ; Re-Start Timer B 
 STY $DC0F      ; Force latched value to be loaded to Timer B counter 
                ; Timer B counts microprocessor cycles 

 INC $D020      

 EOR #$02       ; Revert bit 
 LSR 
 LSR 
 ROR $A9        ; Move it to MSb of $A9 (Endianess: LSbF) 
 BCC done       ; Whole byte read? 
 NOP 
 LDA $DC0D      
 PLA 
 TAY 
 PLA 
 RTI 
 

What this code does is to compare the Timer B Countdown value with $0200. Since the initial value is $03A0 (clock cycles), and it counts DOWN to 0, if the Countdown is at a value greater than $0200 clock cycles when a pulse received on FLAG line(1), the latter is shorter than $01A0 clock cycles. $01A0 is therefore the Threshold value (in clock cycles).

In our example, the TAP value is then:

TAP threshold byte = Threshold (in microseconds) * 0.123156 = $34

where the Threshold (in microseconds) is Threshold * 1e6/CPUFrequency.

Regards,

Luigi.

(1) on CIA #1, this FLAG line is connected to the Cassette Read line of the Cassette Port.