From c9df40c99f40418d869712b31de1b59d7654362f Mon Sep 17 00:00:00 2001 From: alekseiplusplus Date: Mon, 3 Apr 2023 13:36:27 +1000 Subject: removed some files --- resources/Assembly In One Step.html | 938 ------------------------------------ 1 file changed, 938 deletions(-) delete mode 100755 resources/Assembly In One Step.html (limited to 'resources/Assembly In One Step.html') diff --git a/resources/Assembly In One Step.html b/resources/Assembly In One Step.html deleted file mode 100755 index 2a57ad1..0000000 --- a/resources/Assembly In One Step.html +++ /dev/null @@ -1,938 +0,0 @@ - - - Assembly In One Step - - - - -

Assembly In One Step

-RTK, last update: 23-Jul-97

- -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. 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.
-                          
-                             
-   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.
-             
-       D  =  DECIMAL MODE. Set if decimal mode active.
-       
-       I  =  IRQ DISABLE.  Set if maskable interrupts are disabled.
-             
-       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
-   --------------------
-   
-   The first 256 memory locations ($0000-00FF) are called "zero page".  The 
-   next 256 instructions ($0100-01FF) are page 1, etc.  Instructions 
-   making use of the zero page save memory by not using an extra $00 to 
-   indicate the high part of the address.  For example,
-   
-      LDA $0023   -- works but uses an extra byte
-      LDA $23     -- the zero page address
-      
-      
-   Implied Addressing
-   ------------------
-   
-   Many instructions are only one byte in length and do not reference 
-   memory.  These are said to be using implied addressing.  For example,
-   
-      CLC  -- Clear the carry flag
-      DEX  -- Decrement the X register by one
-      TYA  -- Transfer the Y register to the accumulator
-      
-      
-   Indirect Absolute Addressing
-   ----------------------------
-   
-   Only used by JMP (JuMP).  It takes the given address and uses it as a 
-   pointer to the low part of a 16-bit address in memory, then jumps to 
-   that address.  For example,
-   
-      JMP ($2345)   -- jump to the address in $2345 low and $2346 high
-      
-      So if $2345 contains $EA and $2346 contains $12 then the next 
-      instruction executed is the one stored at $12EA.  Remember, the 
-      6502 puts its addresses in low/high format.
-   
-   
-   Absolute Indexed Addressing
-   ---------------------------
-   
-   The final address is found by taking the given address as a base and 
-   adding the current value of the X or Y register to it as an offset.  So,
-   
-      LDA $F453,X  where X contains 3
-      
-   Load the accumulator with the contents of address $F453 + 3 = $F456.
-   
-   
-   Zero Page Indexed Addressing
-   ----------------------------
-   
-   Same as Absolute Indexed but the given address is in the zero page 
-   thereby saving a byte of memory.
-   
-   
-   Indexed Indirect Addressing
-   ---------------------------
-   
-   Find the 16-bit address starting at the given location plus the 
-   current X register.  The value is the contents of that address.  For 
-   example,
-   
-      LDA ($B4,X)  where X contains 6
-      
-   gives an address of $B4 + 6 = $BA.  If $BA and $BB contain $12 and 
-   $EE respectively, then the final address is $EE12.  The value at 
-   location $EE12 is put in the accumulator.
-   
-   
-   Indirect Indexed Addressing
-   ---------------------------
-   
-   Find the 16-bit address contained in the given location ( and the one 
-   following).  Add to that address the contents of the Y register.  
-   Fetch the value stored at that address.  For example,
-   
-      LDA ($B4),Y  where Y contains 6
-      
-   If $B4 contains $EE and $B5 contains $12 then the value at memory 
-   location $12EE + Y (6) = $12F4 is fetched and put in the accumulator.
-   
-   
-   Relative Addressing
-   -------------------
-   
-   The 6502 branch instructions use relative addressing.  The next byte 
-   is a signed offset from the current address, and the net sum is the 
-   address of the next instruction executed.  For example,
-   
-      BNE $7F   (branch on zero flag reset)
-      
-   will add 127 to the current program counter (address to execute) and 
-   start executing the instruction at that address.  SImilarly,
-   
-      BEQ $F9   (branch on zero flag set)
-      
-   will add a -7 to the current program counter and start execution at 
-   the new program counter address.
-   
-   Remember, if one treats the highest bit (bit 7) of a byte as a sign (0 
-   = positive, 1 = negative) then it is possible to have numbers in the 
-   range -128 ($80) to +127 (7F).  So, if the high bit is set, i.e. the 
-   number is > $7F, it is a negative branch.  How far is the branch?  If 
-   the value is < $80 (positive) it is simply that many bytes.  If the 
-   value is > $7F (negative) then it is the 2's compliment of the given 
-   value in the negative direction.
-   
-      2's compilment
-      --------------
-      
-      The 2's compilment of a number is found by switching all the bits 
-      from 0 -> 1 and 1 -> 0, then adding 1.  So,
-      
-      $FF  =  1111 1111   <-- original
-              0000 0000   <-- 1's compliment
-           +          1  
-              ---------
-              0000 0001   <-- 2's compliment, therefore $FF = -1
-              
-      Note that QForth uses this for numbers greater than 32768 so that
-      65535 = -1 and 32768 = -32768.
-      
-   In practice, the assembly language programmer uses a label and the 
-   assembler takes care of the actual computation.  Note that branches 
-   can only be to addresses within -128 to +127 bytes from the present 
-   address.  The 6502 does not allow branches to an absolute address.
-   
-   
-   Accumulator Addressing
-   ----------------------
-   
-   Like implied addressing, the object of the instruction is the 
-   accumulator and need not be specified.
-   
-   
-   
-The 6502 Instruction Set
-------------------------
-
-   There are 56 instructions in the 6502, and more in the 65C02.  Many 
-   instructions make use of more than one addressing mode and each 
-   instruction/addressing mode combination has a particular hexadecimal 
-   opcode that specifies it exactly.  So,
-   
-      A9  =  LDA #$aa   Immediate addressing mode load of accumulator
-      AD  =  LDA $aaaa  Absolute addressing mode load of accumulator
-      etc.
-   
-   
-   Some 6502 instructions make use of bitwise logic.  This includes AND, 
-   OR, and EOR (Exclusive-OR).  The tables below illustrate the effects 
-   of these operations:
-   
-      AND   1  1  ->  1    "both"
-            1  0  ->  0
-            0  1  ->  0
-            0  0  ->  0
-            
-      OR    1  1  ->  1    "either one or both"
-            1  0  ->  1
-            0  1  ->  1
-            0  0  ->  0
-            
-      EOR   1  1  ->  0    "one or the other but not both"
-            1  0  ->  1
-            0  1  ->  1
-            0  0  ->  0
-   
-    Therefore,  $FF AND $0F  =  $0F since,
-    
-             1111 1111
-        and  0000 1111
-             ---------
-             0000 1111  = $0F
-   
-   
-   AND is useful for masking bits.  For example, to mask the high order 
-   bits of a value AND with $0F:
-   
-      $36 AND $0F  =  $06
-      
-   OR is useful for setting a particular bit:
-   
-      $80 OR $08   =  $88
-      
-      since  1000 0000  ($80)
-             0000 1000  ($08)
-          or ---------
-             1000 1000  ($88)
-   
-   EOR is useful for flipping bits:
-   
-      $AA EOR $FF  =  $55
-      
-      since  1010 1010  ($AA)
-             1111 1111  ($FF)
-         eor ---------
-             0101 0101  ($55)
-   
-   
-   Other 6502 instructions shift bits to the right or the left or rotate 
-   them right or left.  Note that shifting to the left by one bit is the 
-   same as multipling by 2 and that shifting right by one bit is the same 
-   as dividing by 2.
-   
-   
-   The 6502 instructions fall naturally into 10 groups with two odd-ball 
-   instructions NOP and BRK:
-   
-      Load and Store Instructions
-      Arithmetic Instructions
-      Increment and Decrement Instructions
-      Logical Instructions
-      Jump, Branch, Compare and Test Bits Instructions
-      Shift and Rotate Instructions
-      Transfer Instructions
-      Stack Instructions
-      Subroutine Instructions
-      Set/Reset Instructions
-      NOP/BRK Instructions
-      
-      
-   
-   Load and Store Instructions
-   ===========================
-   
-   LDA  - LoaD the Accumulator
-   LDX  - LoaD the X register
-   LDY  - LoaD the Y register
-   
-   STA  - STore the Accumulator
-   STX  - STore the X register
-   STY  - STore the Y register
-   
-   Microprocessors spend much of their time moving stuff around in 
-   memory.  Data from one location is loaded into a register and stored 
-   in another location, often with something added or subtracted in the 
-   process.  Memory can be loaded directly into the A, X, and Y registers 
-   but as usual, the accumulator has more addressing modes available.
-   
-   If the high bit (left most, bit 7) is set when loaded the N flag on 
-   the processor status register is set.  If the loaded value is zero the 
-   Z flag is set. 
-   
-   
-   Arithmetic Instructions
-   =======================
-   
-   ADC  - ADd to accumulator with Carry
-   SBC  - SuBtract from accumulator with Carry
-   
-   The 6502 has two arithmetic modes, binary and decimal.  Both addition 
-   and subtraction implement the carry flag to track carries and borrows 
-   thereby making multibyte arithmetic simple.  Note that in the case of 
-   subtraction it is necessary to SET the carry flag as it is the opposite 
-   of the carry that is subtracted.
-   
-   Addition should follow this form:
-   
-   CLC
-   ADC ...    
-   .
-   .
-   ADC ...
-   .
-   .
-   .
-   
-   Clear the carry flag, and perform all the additions.  The carry 
-   between additions will be handled in the carry flag.  Add from low 
-   byte to high byte.  Symbolically, the net effect of an ADC instruction is:
-   
-   A + M + C  -->  A
-   
-   
-   Subtraction follows the same format:
-   
-   SEC
-   SBC ...
-   .
-   .
-   SBC ...
-   .
-   .
-   .
-   
-   In this case set the carry flag first and then do the subtractions.  
-   Symbolically,
-   
-   A - M - ~C  -->  A  ,  where ~C is the opposite of C
-   
-   
-   Ex.1
-   ----
-        A 16-bit addition routine.  $20,$21 + $22,$23 = $24,$25
-        
-           CLC         clear the carry
-           LDA $20     get the low byte of the first number
-           ADC $22     add to it the low byte of the second
-           STA $24     store in the low byte of the result
-           LDA $21     get the high byte of the first number
-           ADC $23     add to it the high byte of the second, plus carry
-           STA $25     store in high byte of the result
-           
-           ... on exit the carry will be set if the result could not be
-               contained in 16-bit number.
-               
-   Ex.2
-   ----
-        A 16-bit subtraction routine.  $20,$21 - $22,$23 = $24,$25
-        
-           SEC         clear the carry
-           LDA $20     get the low byte of the first number
-           SBC $22     add to it the low byte of the second
-           STA $24     store in the low byte of the result
-           LDA $21     get the high byte of the first number
-           SBC $23     add to it the high byte of the second, plus carry
-           STA $25     store in high byte of the result
-           
-           ... on exit the carry will be set if the result produced a 
-               borrow
-               
-   Aside from the carry flag, arithmetic instructions also affect the N, 
-   Z, and V flags as follows:
-   
-      Z = 1  if result was zero, 0 otherwise
-      N = 1  if bit 7 of the result is 1, 0 otherwise
-      V = 1  if bit 7 of the accumulator was changed, a sign change
-      
-      
-   
-   Increment and Decrement Instructions
-   ====================================
-   
-   INC  - INCrement memory by one
-   INX  - INcrement X by one
-   INY  - INcrement Y by one
-
-   DEC  - DECrement memory by one
-   DEX  - DEcrement X by one
-   DEY  - DEcrement Y by one
-
-   The 6502 has instructions for incrementing/decrementing the index 
-   registers and memory.  Note that it does not have instructions for 
-   incrementing/decrementing the accumulator.  This oversight was 
-   rectified in the 65C02 which added INA and DEA instructions.  The 
-   index register instructions are implied mode for obvious reasons while 
-   the INC and DEC instructions use a number of addressing modes.
-   
-   All inc/dec instructions have alter the processor status flags in the 
-   following way:
-   
-     Z = 1  if the result is zero, 0 otherwise
-     N = 1  if bit 7 is 1, 0 otherwise
-     
-   
-   
-   Logical Instructions
-   ====================
-   
-   AND  - AND memory with accumulator
-   ORA  - OR memory with Accumulator
-   EOR  - Exclusive-OR memory with Accumulator
-   
-   These instructions perform a bitwise binary operation according to the 
-   tables given above.  They set the Z flag if the net result is zero and 
-   set the N flag if bit 7 of the result is set.
-   
-   
-   
-   Jump, Branch, Compare, and Test Bits
-   ====================================
-   
-   JMP  - JuMP to another location (GOTO)
-   
-   BCC  - Branch on Carry Clear,       C = 0
-   BCS  - Branch on Carry Set,         C = 1
-   BEQ  - Branch on EQual to zero,     Z = 1
-   BNE  - Branch on Not Equal to zero, Z = 0
-   BMI  - Branch on MInus,             N = 1
-   BPL  - Branch on PLus,              N = 0
-   BVS  - Branch on oVerflow Set,      V = 1
-   BVC  - Branch on oVerflow Clear,    V = 0
-   
-   CMP  - CoMPare memory and accumulator
-   CPX  - ComPare memory and X
-   CPY  - ComPare memory and Y
-   
-   BIT  - test BITs
-   
-   This large group includes all instructions that alter the flow of the 
-   program or perform a comparison of values or bits.
-   
-   JMP simply sets the program counter (PC) to the address given.  
-   Execution proceeds from the new address.  The branch instructions are 
-   relative jumps.  They cause a branch to a new address that is either 
-   127 bytes beyond the current PC or 128 bytes before the current PC.  
-   Code that only uses branch instructions is relocatable and can be run 
-   anywhere in memory.
-   
-   The three compare instructions are used to set processor status bits.  
-   After the comparison one frequently branches to a new place in the 
-   program based on the settings of the status register.  The 
-   relationship between the compared values and the status bits is,
-   
-   
-          +-------------------------+---------------------+
-          |                         |  N       Z       C  |
-          +-------------------------+---------------------+
-          | A, X, or Y  <  Memory   |  1       0       0  |
-          | A, X, or Y  =  Memory   |  0       1       1  |
-          | A, X, or Y  >  Memory   |  0       0       1  |
-          +-----------------------------------------------+
-          
-          
-   The BIT instruction tests bits in memory with the accumulator but 
-   changes neither.  Only processor status flags are set.  The contents 
-   of the specified memory location are logically ANDed with the 
-   accumulator, then the status bits are set such that,
-   
-   * N receives the initial, un-ANDed value of memory bit 7.
-   * V receives the initial, un-ANDed value of memory bit 6.
-   * Z is set if the result of the AND is zero, otherwise reset.
-   
-   So, if $23 contained $7F and the accumulator contained $80 a BIT $23 
-   instruction would result in the V and Z flags being set and N reset since 
-   bit 7 of $7F is 0, bit 6 of $7F is 1, and $7F AND $80 = 0.
-   
-   
-   
-   Shift and Rotate Instructions
-   =============================
-   
-   ASL  - Accumulator Shift Left
-   LSR  - Logical Shift Right
-   ROL  - ROtate Left
-   ROR  - ROtate Right
-   
-   Use these instructions to move things around in the accumulator or 
-   memory.  The net effects are (where C is the carry flag):
-
-   
-           +-+-+-+-+-+-+-+-+ 
-      C <- |7|6|5|4|3|2|1|0| <- 0    ASL
-           +-+-+-+-+-+-+-+-+
-           
-           +-+-+-+-+-+-+-+-+ 
-      0 -> |7|6|5|4|3|2|1|0| -> C    LSR
-           +-+-+-+-+-+-+-+-+
-
-           +-+-+-+-+-+-+-+-+ 
-      C <- |7|6|5|4|3|2|1|0| <- C    ROL
-           +-+-+-+-+-+-+-+-+
-
-           +-+-+-+-+-+-+-+-+ 
-      C -> |7|6|5|4|3|2|1|0| -> C    ROR
-           +-+-+-+-+-+-+-+-+
-
-
-    Z is set if the result it zero.  N is set if bit 7 is 1.  It is 
-    always reset on LSR.  Remember that ASL A is equal to multiplying by 
-    two and that LSR is equal to dividing by two.
-    
-    
-    
-    Transfer Instructions
-    =====================
-    
-    TAX  - Transfer Accumulator to X
-    TAY  - Transfer Accumulator to Y
-    TXA  - Transfer X to accumulator
-    TYA  - Transfer Y to Accumulator
-    
-    Transfer instructions move values between the 6502 registers.  The N 
-    and Z flags are set if the value being moved warrants it, i.e.
-    
-    LDA #$80
-    TAX
-    
-    causes the N flag to be set since bit 7 of the value moved is 1, while
-    
-    LDX #$00
-    TXA
-    
-    causes the Z flag to be set since the value is zero.
-    
-    
-    
-    Stack Instructions
-    ==================
-    
-    TSX  - Transfer Stack pointer to X
-    TXS  - Transfer X to Stack pointer
-
-    PHA  - PusH Accumulator on stack
-    PHP  - PusH Processor status on stack
-    PLA  - PulL Accumulator from stack
-    PLP  - PulL Processor status from stack
-    
-    TSX and TXS make manipulating the stack possible.  The push and pull 
-    instructions are useful for saving register values and status flags.  
-    Their operation is straightforward.
-    
-    
-    
-    Subroutine Instructions
-    =======================
-    
-    JSR  - Jump to SubRoutine
-    RTS  - ReTurn from Subroutine
-    RTI  - ReTurn from Interrupt
-    
-    Like JMP, JSR causes the program to start execution of the next 
-    instruction at the given address.  Unlike JMP, JSR pushes the address 
-    of the next instruction after itself on the stack.  When an RTS 
-    instruction is executed the address pushed on the stack is pulled off 
-    the stack and the program resumes at that address.  For example,
-    
-    LDA #$C1   ; load the character 'A'
-    JSR print  ; print the character and it's hex code
-    LDA #$C2   ; load 'B'
-    JSR print  ; and print it
-    .
-    .
-    .
- print JSR $FDED  ; print the letter
-       JSR $FDDA  ; and its ASCII code
-       RTS        ; return to the caller
-    
-    RTI is analagous to RTS and should be used to end an interrupt routine.
-    
-    
-    
-    Set and Reset (Clear) Instructions
-    ==================================
-    
-    CLC  - CLear Carry flag
-    CLD  - CLear Decimal mode
-    CLI  - CLear Interrupt disable
-    CLV  - CLear oVerflow flag
-    
-    SEC  - SEt Carry
-    SED  - SEt Decimal mode
-    SEI  - SEt Interrupt disable
-    
-    These are one byte instructions to specify processor status flag 
-    settings.
-    
-    CLC and SEC are of particular use in addition and subtraction 
-    respectively.  Before any addition (ADC) use CLC to clear the carry 
-    or the result may be one greater than you expect.  For subtraction 
-    (SBC) use SEC to ensure that the carry is set as its compliment is 
-    subtracted from the answer.  In multi-byte additions or subtractions 
-    only clear or set the carry flag before the initial operation.  For 
-    example, to add one to a 16-bit number in $23 and $24 you would write:
-    
-    LDA $23     ; get the low byte
-    CLC         ; clear the carry
-    ADC #$02    ; add a constant 2, carry will be set if result > 255
-    STA $23     ; save the low byte
-    LDA $24     ; get the high byte
-    ADC #$00    ; add zero to add any carry that might have been set above
-    STA $24     ; save the high byte
-    RTS         ; if carry set now the result was > 65535
-    
-    Similarly for subtraction,
-    
-    LDA $23     ; get the low byte
-    SEC         ; set the carry
-    SBC #$02    ; subtract 2
-    STA $23     ; save the low byte
-    LDA $24     ; get the high byte
-    SBC #$00    ; subtract 0 and any borrow generated above
-    STA $24     ; save the high byte
-    RTS         ; if the carry is not set the result was < 0
-    
-    
-    
-    Other Instructions
-    ==================
-    
-    NOP  - No OPeration (or is it NO oPeration ? :)
-    BRK  - BReaK
-    
-    NOP is just that, no operation.  Useful for deleting old 
-    instructions, reserving room for future instructions or for use in 
-    careful timing loops as it uses 2 microprocessor cycles.
-    
-    BRK causes a forced break to occur and the processor will immediately 
-    start execution of the routine whose address is in $FFFE and $FFFF.  
-    This address is often the start of a system monitor program.
-    
-    
-    
-Some simple programming examples
-================================
-
-    A few simple programming examples are given here.  They serve to 
-    illustrate some techniques commonly used in assembly programming.  
-    There are doubtless dozens more and I make no claim at being a 
-    proficient assembly language programmer.  For examples of addition 
-    and subtraction see above on CLC and SEC.
-    
-    
-    A count down loop
-    -----------------
-    
-            ; 
-            ; An 8-bit count down loop
-            ;
-            
-            start LDX #$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. :)
-
-
-
-Last update: 30-Jan-00
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