How to toggle 1 bit and set 1 bit to 0 in assembly with only XOR,OR,AND?

Question

Currently I'm using a chip from school which has a limited number of instructions. What I want to achieve is first toggling a single bit with mask and then set some bits to zero. Only 3 instructions available: AND, OR, XOR. (no SHIFT NOT instruction)

for example

0001 1001   // toggle bit 5
0000 1001   
0001 1001   // set bit 1 to 0
0001 1000

I'm trying to generate PWM with H-bridge.

Edit:

Fixed:

; toggle bits
LOAD    R0 [GB+OUTPUT_BUF]
XOR     R0 CONV_FORW_MASK
XOR     R0 FLIP_FORW_MASK
XOR     R0 PUSH_FORW_MASK
XOR     R0 PUSH_BACK_MASK

; set mask bit to zero  
LOAD    R1 PUSH_FORW_MASK
XOR     R1 -1
AND     R0 [R1]

goto wikipedia and look up the truth table for xor, and and or, the truth table will explain everything. — old_timer, Jun 10 '14 at 14:44
use `xor` to toggle, `and` to clear, `or` to set bit. No usage of `not` here — phuclv, Jun 10 '14 at 15:37
`not` is also a subtraction from -1 or 0xFFFF... like in this question http://stackoverflow.com/questions/24107095/flipping-bits-in-an-integer?noredirect=1#comment37189022_24107095 — phuclv, Jun 10 '14 at 16:37
possible duplicate of [How do you set, clear and toggle a single bit in C/C++?](http://stackoverflow.com/questions/47981/how-do-you-set-clear-and-toggle-a-single-bit-in-c-c) — phuclv, Feb 28 '15 at 06:44

score 1 · Answer 1 · answered Jun 10 '14 at 15:42

1

actually, one instruction, NAND (which is AND with result inverted) would be sufficient, as you can build an XOR (hint, hint) from 4 of those. I'd suggest to start exercising by trying to build an XOR from NANDs only. Then the rest may just fall into place by itself.

answered Jun 10 '14 at 15:42

Deleted User

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with only a NAND or a NOR instruction you can do any logic functions http://en.wikipedia.org/wiki/NAND_logic http://en.wikipedia.org/wiki/NOR_logic – phuclv Jun 11 '14 at 02:08

score 1 · Accepted Answer · answered Jun 10 '14 at 16:19

1

In C it will be :

unsigned char byte = 0x19;

unsigned char bit_set = (0x01 & byte) ^ byte;

unsigned char bit_toggle = ((byte ^ 0x10) & 0x10);

byte = ( byte & 0xEF ) | bit_toggle ;

Now translate to assembly.

answered Jun 10 '14 at 16:19

lsalamon

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to set bit, use `byte | 0x01` instead, simpler and faster. to clear it, use `byte & 0xfe`. to toggle, use `byte ^ 0x01` – phuclv Jun 11 '14 at 02:10
for the toggling, the final `&` is completely redundant – phuclv Feb 28 '15 at 06:45

How to toggle 1 bit and set 1 bit to 0 in assembly with only XOR,OR,AND?

Edit:

2 Answers2