Bit Shifting
The 6502 has 2 ways of shifting bits left and right. In these examples, I will number each bit…there are 8 bits, numbered 0-7. Only the accumulator (A register) can do bit shifts and bitwise operations. Also, you can do bit shifting to a RAM address, without affecting A.
LSR – shift right
zero -> 76543210 -> carry flag
ROR – roll right
old carry flag -> 76543210 -> new carry flag
ASL – shift left
carry flag <- 76543210 <- zero
ROL – roll left
new carry flag <- 76543210 <- old carry flag
LSR shifts all the bits right one position, and a zero goes into the highest bit. Effectively, this is the same as dividing by 2, with some rounding error.
zero -> 00010000
00001000, carry = 0
zero -> 00001111
00000111, carry = 1
ROR works the same as LSR, except the old carry flag goes in rather than zero.
ASL shifts all the bits left one position, and a zero goes into the lowest bit. Effectively, this is the same as multiplying by 2. Right to left here…
00010000 <- zero <- 00100000 carry = 0
11110000 <- zero <- 11100000 carry = 1
ROL works the same as ASL, except the old carry flag goes in rather than zero.
Now, some examples, using C programming examples to start with.
foo = bar << 2; // 8-bit numbers
LDA bar load A from address bar ASL A bit-shift A left ASL A bit-shift A left STA foo store A to address foo foo = bar >> 3; //8-bit numbers
LDA bar load A from address bar LSR A bit-shift A right LSR A bit-shift A right LSR A bit-shift A right STA foo store A to address foo And, here's some 16-bit examples.
foo = bar << 2; // 16-bit numbers
LDA bar+1 load A from address bar+1 (the high byte) STA foo+1 store A to a address foo+1 (the high byte) LDA bar lda A from address bar (the low byte) ASL A bit-shift A left (high bit shifted into carry flag) ROL foo+1 bit-shift left address 'foo+1', rolling that carry flag in ASL A bit-shift A left (high bit shifted into carry flag) ROL foo+1 bit-shift left address 'foo+1', rolling that carry flag in STA foo store A to the address foo (the low byte)
foo = bar >> 3; //16-bit numbers
LDA bar load A from address bar (the low byte) STA foo store A to the address foo (the low byte) LDA bar+1 load A from the address bar+1 (the high byte) LSR A bit-shift A right (low bit shifted into carry flag) ROR foo bit shift right address 'foo', rolling that carry flag in LSR A ... ROR foo LSR A ROR foo ...3 times STA foo+1 store A to the address foo+1 (the high byte).
Bitwise Operations.
AND, OR, and XOR…called here AND, ORA, and EOR. These things only work with the A register.
Here’s what AND does. bit by bit.
AND #value
A, AND value= result
0 AND 0 = 0 0 AND 1 = 0 1 AND 0 = 0 1 AND 1 = 1
AND only sets a bit if both bits in A and value are 1.
Example:
A 00010001 value 00000101 result 00000001
AND is used to isolate a single bit. The way I handle button presses, I roll them into joypad1. If I want to find out if the Left button is being pressed…I know that it is this bit of joypad1 00000010 ($02). Here’s how the code would usually go…
LEFT = $02 ;defined at the top of the page
LDA joypad1 load A from address joypad1
AND #LEFT AND A with value 2, result now in A
BEQ :+ branch if the result is zero (to unnamed label)
skipping this next line of code
JSR Left_Pressed jump to sub-routine handline left button presses
: just a label
Another use for AND, is to ‘mask’ out certain bits. Let’s say, I have a peice of data, where the upper bit is a special flag, and the lower 7 bits is the data. If I want just the data, I would AND #$7f (01111111) to remove the upper-bit…
LDA data AND #$7f
ORA (bitwise OR operation)
Here’s what ORA does. bit by bit.
ORA #value A, ORA value= result 0 ORA 0 = 0 0 ORA 1 = 1 1 ORA 0 = 1 1 ORA 1 = 1
If either A or the value has a bit set, the result will have that bit set.
Example: A 00010001 value 00000101 result 00010101
ORA is a way ensure that certain bits are set, without effecting the other bits (as math would do).
Music code is a good example. The left 4 bits control the sound. The right 4 bits volume. So, if you want to keep the ‘instrument’ the same, the first 4 bits would always be $C (for example), while the volume may change. So, you might store $c0 in variable ‘instrument’ and store the volume in variable ‘volume’. When you need to combine them, you would use ORA.
LDA instrument instrument is $c0 ORA volume volume is 0 - $0f STA $4000 result stored to music register, address $4000.
EOR (exclusive OR operation), means one or the other, but not both.
EOR #value A, EOR value= result 0 EOR 0 = 0 0 EOR 1 = 1 1 EOR 0 = 1 1 EOR 1 = 0
Example: A 00010001 value 00000101 result 00010100
EOR is usually used to get the negative value of a number. Say you have -5 ($fb) and you want to turn it into 5, you EOR #$ff and add 1. The same for converting back to -5.
LDA foo Let's say foo = $fb (-5) EOR #$ff A = 4 now CLC ADC #1 A = 5 now
and reverse works too…
LDA foo Let's say foo = 5 EOR #$ff A = $fa now CLC ADC #1 A = $fb now (-5)
TRANSFERRING registers
TAX A transfers to X
TXA X transfers to A
TAY A transfers to Y
TYA Y transfers to A
TXS X transferred to the stack pointer
TSX stack pointer transferred to X
This is the only way to access the stack pointer. Usually, the stack pointer is set to $ff at the start of the program and never thought of again.
LDX #$ff load X with value $ff TXS transfer to stack pointer
(the stack grows down from $1ff)
More Stack Operations
PHA push A to the stack (and adjust the stack pointer -1)
PLA pull A (pop A) from the stack (and adjust the stack pointer +1)
PHP push the processor status to the stack (and stack pointer -1)
PLP pull the processor status from the stack (and stack pointer +1)
Unfortunately, you can’t push a few arguments to the stack, jump to a sub-routine and then use those numbers…not easily, at least. Because, the jump to sub-routine also pushes the return address to the stack, on top of your numbers. PHA and PLA can be used as a cheap local variable. But, be careful. If you’re inside a sub-routine and you PHA, and forget to PLA, your program will crash when it tries to pull the return address, and gets your PHA number instead.
A few more things…
NOP does nothing but wastes 2 cycles of CPU time
BRK a non-maskable interrupt…will jump the program to wherever the BRK vector tells it…this usually only happens if a big error has occured, as the machine code for BRK is #00…which indicates that the program has branched to an area of the ROM with nothing there.
And, next time we will go over jumping, branching, and comparison.
nesdoug 