Assembly Language Subroutines Lance A. Leventhal Winthrop Saville OSBORNE/McGraw-Hill Berkeley, California Disclaimer of Warranties and. Assembly Language Subroutines. The authors and the publisher apparently assume that if you don’t know which computers use the microprocessor. assembly language subroutines / Lance A. Leventhal, Winthrop Saville. Subjects: Assembly languages (Electronic computers) · (Microprocessor) > .

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We can generalize array processing by storing the base address in two locations on page and using the postindexed indirect indexed addressing mode. A, P Execution Time: Free returns on all UK orders Buy with confidence. If you assign different areas of memory for each call or each routine, you are essentially creating your own stack.

The program provides auto- matic address wraparound mod 64Kbut the results of any move asswmbly the pro- gram’s own temporary storage are unpredic- table.

That is, you must save and restore the status register as follows: There is some potential suboutines here — BEQ means branch if the result is equal to 0; that is, branch if the Zero flag is 1. The mnemonic may confuse the programmer, but the microprocessor does exactly what it is supposed to do. Sometimes transferring data to or from a port will change the status lines automatically, particularly if you are using a or parallel interface.

A page glossary completes the book. Clear bit 3 of the accumulator. Note that we could eliminate the BCS instruction from the program without affecting the result, but the change would increase the shbroutines execution subroktines. Stack Loading of Registers The instruction PLA loads the accumulator from the top of the stack and subtracts 1 from the stack pointer.

The other bits are arbitrary, although our preference is to clear unused bits as a stan- dard convention. Amazon Inspire Digital Educational Resources. The following are languagw tive implementations: Customers who bought this item also bought. So the Zero flag ends up containing the logical complement of bit 3. The array is assumed to be stored in row subroufines order that is, organized so that subscripts to the right change before subscripts to the left.


Return from Interrupt Instructions If the initial portion of the interrupt service routine saves all the registers with the sequence. Thus, it is much simpler to increment a multibyte number than to decrement it.

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Instructions that operate on data in memory can also be used for input or out- put. Chapter 6 contains additional examples of arithmetic programs.

The usual way to inform the main program is to have the interrupt service routine change a flag that the main program can examine. A size of zero causes an immediate exit with no memory changed. If that flag is 0, the Negative flag indicates which operand is larger; if that flag is 1, the sense of the Negative flag is inverted.

Transfer accumulator to status register.

6502 assembly language subroutines

Search the history of over billion web pages on the Internet. Note that comparing equal operands clears the Negative flag. The authors and the publisher apparently assume that if you don’t know which xssembly use the microprocessor, you wouldn’t be interested in this book, which doesn’t reveal until page 54 that the is used in the Apple.

Load status register direct from memory location ADDR.

We are a specialist Oxfam books and music shop. Once the pro- cessor has read data from an input port, that port should revert to the not ready state.

Unless the misspelling is invalid, the assembler has no way of knowing what you meant. You should remember that RTS is simply a jump lanhuage that obtains the new value for the program counter from the top of the stack. We have aimed this book at the person who wants to use assembly language immediately, rather than just learn about it.


Reading T2 low-order counter or writing T2 high-order counter. Examples 1 Add an element of an array to the accumulator.

The indirect address is in two memory locations starting at the specified address; it is stored upside-down, with its less significant byte at the lower address. Approximately 11 cycles per byte plus 93 cycles overhead. The programmer must make sure that all memory locations used indirectly contain addresses with the bytes arranged properly.

Decimal add memory location ADDR to accumulator. END Subroutinse of program. Note that skip instructions are awkward to implement on most microprocessors, because their instructions vary in length and it is difficult to determine how long a jump is required to skip an instruction.

The other flags are also changed. Store index register in stack. You will surely have problems if you try to use the jump instructions as indirect addresses or if you try to execute the indirect addresses. Of course, input and lanbuage are in some sense mirror images. If you read some basics of the hardware you want to work on, this book will take your work to new heights.

Note that the Carry is the inverse of a normal borrow.

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So, the following sequences implement decimal addition and subtraction: String consists of Result: Indexed addressing is available for most instructions in the set.

TheNorthern Technology Books, Evanston, Return to Step 2 if the result of Step 3 is not 0. Asxembly the two operands are equal, the Carry is set to 1, just as if the accumulator were larger.