Assembly language programming for the Atari computers by Mark Chasin

By Mark Chasin

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The value of the decimal flag, D, in the processor status register determines which mode the processor is in. If this value is 1, all operations will be in the decimal mode, and if it is 0, they will be binary. In general, most operations in assembly language use binary math, but you have the ability to switch by toggling this flag. The ATARI Hardware 31 THE BREAK FLAG The break flag, or B flag, can be set and cleared only by the 6502 itself. The B flag cannot be altered by the programmer. It is used to determine whether an interrupt was caused by the 6502 instruction BRK, which stands for BReaK.

In our discussion so far, we have concentrated entirely on positive numbers. How do we handle negative numbers? By using signed binary arithmetic. In this system, the left-hand-most bit, called the most significant bit, doesn't represent a power of 2 at all, but rather represents the sign of the number. That is, if the most signficiant bit is 1, the number is negative, and if the most significant bit is zero, the number is positive (Fig. 2-2). One fallout of this system is that the largest signed number we can represent in 1 byte is + 128, or - 127, since we only have 7 arithmetic bits with which to work .

In this example, we could check in line 30 to see if FILE = 33 by checking the value of I. If 1=0, then we know that FILE doesn't equal 33, but if 1= 1, then we know that FILE = 33. In this example, I is a flag which gives us information about the value of FILE. In much the same way the seven flags in the processor status register of the 6502 give us considerable information about what's happening during our program. Each flag is a single bit in the single byte of this register (Fig. 3-3). Flags are usually known by their single-letter abbreviations, as follows: Letter Flag Meaning C Z I D B V N Carry Zero IRQ disable Decimal mode Break command Overflow Negative 1 = true = resu lt of zero = disable = true = true = negative The ATARI Hardware 29 ti"~ Nega Over flow Future use Break command Decim al Inter rupt disable Zero Carry Fig.

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