# Atari assembly language programmer's guide by Allan E. Moose, Marian J. Lorenz

By Allan E. Moose, Marian J. Lorenz

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Decimal, binary, a nd hexa decimal systems. I DECIMAL BINARY HEX 0 1 2 3 0000 0001 0010 0011 0100 0101 0110 0111 1000 1001 1010 1011 1100 1101 1110 1111 0 1 2 3 4 5 4 5 6 7 8 9 10 11 12 13 14 15 6 7 8 9 A B C D E F 32 Atari Assembly Language Programme(s Guide Conversion of a binary number into hexadecimal is straight forward. Separate it into its groups of four bits and write the hex equivalent of each group: 10 1" - .. 1 .... \-1- 1-- = AF or Conversion from hexadecimal to binary is equally simple: A9 = 1 01 0 t 1 00 1 t 9 A Conversion from hexadecimal to decimal can be done using the recursive rule [previous result*base]+[next digit]=result rule.

Indexed indirect Overview of 6502 Instructions 57 Immediate Addressing Mode: The immediate addressing mode takes its operand from the memory location immediately following the op-code . Therefore , it is a two-byte in struction; one byte of op-code followed by a one byte operand. Absolute Addressing Mode: In the absolute addressing mode, the two bytes following the op-code give the memory address from which the CPU is to fetch data to be operated on , or where a byte of data is to be stored . The absolute addressing mode has the following format: First byte Second byte Third Byte Op-code La-Byte of addr.

This flag is used to signal the processor that addition and subtraction are to be performed in the decimal mode using BCD . B == Break Flag. This flag is set, along with the I flag whenever a BRK instruction is executed. It is cleared following an RTI instruction. V == Overflow Flag. This flag is set when an addition or subtraction prod uces a resu It greater than 127 or less than -128. It is used in applications involving signed numbers. N == Negative Flag. This flag indicates whether or not the result of a signed arithmetic operation produced a negative result.