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Text File | 1995-03-20 | 32.2 KB | 331 lines | [TEXT/ALFA]

Assembly in one step -------------------- A brief guide to programming the 6502 in assembly language. It will introduce the 6502 architecture, addressing modes, and instruction set. No prior assembly language programming is assumed, however it is assumed that you are somewhat familiar with hexadecimal numbers. Programming examples are given at the end. After reading this it is recommended that you review the file 6502 INSTRUCTION SUMMARY. Much of this material comes from _6502 Software Design_ by Leo Scanlon, Blacksburg, 1980. ================================================================================ The 6502 Architecture --------------------- The 6502 is an 8-bit microprocessor that follows the memory oriented design philosophy of the Motorola 6800. Several engineers left Motorola and formed MOS Technology which introduced the 6502 in 1975. The 6502 gained in popularity because of it's low price and became the heart of several early personal computers including the Apple II, Commodore 64, and Atari 400 and 800. Simplicity is key ----------------- The 6502 handles data in its registers, each of which holds one byte (8-bits) of data. There are a total of three general use and two special purpose registers: accumulator (A) - Handles all arithmetic and logic. The real heart of the system. X and Y - General purpose registers with limited abilities. S - Stack pointer. P - Processor status. Holds the result of tests and flags. All of the 6502 is implemented in MacQForth with the exception of interrupts and decimal mode arithmetic. Stack Pointer ------------- When the microprocessor executes a JSR (Jump to SubRoutine) instruction it needs to know where to return when finished. The 6502 keeps this information in low memory from $0100 to $01FF and uses the stack pointer as an offset. The stack grows down from $01FF and makes it possible to nest subroutines up to 128 levels deep. Not a problem in most cases. Processor Status ---------------- The processor status register is not directly accessible by any 6502 instruction. Instead, there exist numerous instructions that test the bits of the processor status register. The flags within the register are: bit -> 7 0 +---+---+---+---+---+---+---+---+ | N | V | | B | D | I | Z | C | <-- flag, 0/1 = reset/set +---+---+---+---+---+---+---+---+ N = NEGATIVE. Set if bit 7 of the accumulator is set. V = OVERFLOW. Set if the addition of two like-signed numbers or the subtraction of two unlike-signed numbers produces a result greater than +127 or less than -128. B = BRK COMMAND. Set if an interrupt caused by a BRK, reset if caused by an external interrupt. Not used in MacQForth. D = DECIMAL MODE. Set if decimal mode active. Not used in MacQForth. I = IRQ DISABLE. Set if maskable interrupts are disabled. Not used in MacQForth. Z = ZERO. Set if the result of the last operation (load/inc/dec/ add/sub) was zero. C = CARRY. Set if the add produced a carry, or if the subtraction produced a borrow. Also holds bits after a logical shift. Accumulator ----------- The majority of the 6502's business makes use of the accumulator. All addition and subtraction is done in the accumulator. It also handles the majority of the logical comparisons (is A > B ?) and logical bit shifts. X and Y ------- These are index registers often used to hold offsets to memory locations. They can also be used for holding needed values. Much of their use lies in supporting some of the addressing modes. Addressing Modes ---------------- The 6502 has 13 addressing modes, or ways of accessing memory. The 65C02 introduces two additional modes. They are: +---------------------+--------------------------+ | mode | assembler format | +=====================+==========================+ | Immediate | #aa | | Absolute | aaaa | | Zero Page | aa | Note: | Implied | | | Indirect Absolute | (aaaa) | aa = 2 hex digits | Absolute Indexed,X | aaaa,X | as $FF | Absolute Indexed,Y | aaaa,Y | | Zero Page Indexed,X | aa,X | aaaa = 4 hex | Zero Page Indexed,Y | aa,Y | digits as | Indexed Indirect | (aa,X) | $FFFF | Indirect Indexed | (aa),Y | | Relative | aaaa | Can also be | Accumulator | A | assembler labels +---------------------+--------------------------+ (Table 2-3. _6502 Software Design_, Scanlon, 1980) Immediate Addressing -------------------- The value given is a number to be used immediately by the instruction. For example, LDA #$99 loads the value $99 into the accumulator. Absolute Addressing ------------------- The value given is the address (16-bits) of a memory location that contains the 8-bit value to be used. For example, STA $3E32 stores the present value of the accumulator in memory location $3E32. Zero Page Addressing --------------p ----------------- ; ; An 8-bit count down loop ; start LDX #$FF ; load X with $FF = 255 loop DEX ; X = X - 1 BNE loop ; if X not zero then goto loop RTS ; return How does the BNE instruction know that X is zero? It doesn't, all it knows is that the Z flag is set or reset. The DEX instruction will set the Z flag when X is zero. ; ; A 16-bit count down loop ; start LDY #$FF ; load Y with $FF loop1 LDX #$FF ; load X with $FF loop2 DEX ; X = X - 1 BNE loop2 ; if X not zero goto loop2 DEY ; Y = Y - 1 BNE loop1 ; if Y not zero goto loop1 RTS ; return There are two loops here, X will be set to 255 and count to zero for each time Y is decremented. The net result is to count the 16-bit number Y (high) and X (low) down from $FFFF = 65535 to zero. Other examples -------------- ** Note: All of the following examples are lifted nearly verbatim from the book "6502 Software Design", whose reference is above. ; Example 4-2. Deleting an entry from an unordered list ; ; Delete the contents of $2F from a list whose starting ; address is in $30 and $31. The first byte of the list ; is its length. ; deluel LDY #$00 ; fetch element count LDA ($30),Y TAX ; transfer length to X LDA $2F ; item to delete nextel INY ; index to next element CMP ($30),Y ; do entry and element match? BEQ delete ; yes. delete element DEX ; no. decrement element count BNE nextel ; any more elements to compare? RTS ; no. element not in list. done ; delete an element by moving the ones below it up one location delete DEX ; decrement element count BEQ deccnt ; end of list? INY ; no. move next element up LDA ($30),Y DEY STA ($30),Y INY JMP delete deccnt LDA ($30,X) ; update element count of list SBC #$01 STA ($30,X) RTS ; Example 5-6. 16-bit by 16-bit unsigned multiply ; ; Multiply $22 (low) and $23 (high) by $20 (low) and ; $21 (high) producing a 32-bit result in $24 (low) to $27 (high) ; mlt16 LDA #$00 ; clear p2 and p3 of product STA $26 STA $27 LDX #$16 ; multiplier bit count = 16 nxtbt LSR $21 ; shift two-byte multiplier right ROR $20 BCC align ; multiplier = 1? LDA $26 ; yes. fetch p2 CLC ADC $22 ; and add m0 to it STA $26 ; store new p2 LDA $27 ; fetch p3 ADC $23 ; and add m1 to it align ROR A ; rotate four-byte product right STA $27 ; store new p3 ROR $26 ROR $25 ROR $24 DEX ; decrement bit count BNE nxtbt ; loop until 16 bits are done RTS ; Example 5-14. Simple 16-bit square root. ; ; Returns the 8-bit square root in $20 of the ; 16-bit number in $20 (low) and $21 (high). The ; remainder is in location $21. sqrt16 LDY #$01 ; lsby of first odd number = 1 STY $22 DEY STY $23 ; msby of first odd number (sqrt = 0) again SEC LDA $20 ; save remainder in X register TAX ; subtract odd lo from integer lo SBC $22 STA $20 LDA $21 ; subtract odd hi from integer hi SBC $23 STA $21 ; is subtract result negative? BCC nomore ; no. increment square root INY LDA $22 ; calculate next odd number ADC #$01 STA $22 BCC again INC $23 JMP again nomore STY $20 ; all done, store square root STX $21 ; and remainder RTS This is based on the observation that the square root of an integer is equal to the number of times an increasing odd number can be subtracted from the original number and remain positive. For example, 25 - 1 1 -- 24 - 3 2 -- 21 - 5 3 -- 16 - 7 4 -- 9 - 9 5 = square root of 25 -- 0 If you are truly interested in learning more, go to your public library and seek out an Apple machine language programming book. If your public library is like mine, there will still be plenty of early 80s computer books on the shelves. :)