Assembly In One Step

From 970f76b28ff07206b9e7bdfdce84a7bf8f5aecd0 Mon Sep 17 00:00:00 2001 From: alekseiplusplus Date: Tue, 28 Mar 2023 11:43:51 +1100 Subject: Added local resources --- resources/Assembly In One Step.html | 938 ++++++++++++++++++++++++++++++++++++ 1 file changed, 938 insertions(+) create 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 new file mode 100755 index 0000000..2a57ad1 --- /dev/null +++ b/resources/Assembly In One Step.html @@ -0,0 +1,938 @@ + + + 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
+Back + + + \ No newline at end of file -- cgit v1.2.3