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Arduboy2
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Change drawPixel() to avoid Z reg corruption 

Also use _BV() to specify bit masks in the bitshift_left array,
and other minor formatting changes
This commit is contained in:
Scott Allen 2017-06-16 18:05:19 -04:00
parent 6074f9fc56
commit bd5b470294
1 changed files with 15 additions and 9 deletions
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24
src/Arduboy2.cpp
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@ -276,8 +276,8 @@ void Arduboy2Base::clear()
// Used by drawPixel to help with left bitshifting since AVR has no // Used by drawPixel to help with left bitshifting since AVR has no
// multiple bit shift instruction. We can bit shift from a lookup table // multiple bit shift instruction. We can bit shift from a lookup table
// in flash faster than we can calculate the bit shifts on the CPU. // in flash faster than we can calculate the bit shifts on the CPU.
PROGMEM const unsigned char bitshift_left[] = { const uint8_t bitshift_left[] PROGMEM = {
1, 2, 4, 8, 16, 32, 64, 128 _BV(0), _BV(1), _BV(2), _BV(3), _BV(4), _BV(5), _BV(6), _BV(7)
}; };
void Arduboy2Base::drawPixel(int16_t x, int16_t y, uint8_t color) void Arduboy2Base::drawPixel(int16_t x, int16_t y, uint8_t color)
@ -299,8 +299,13 @@ void Arduboy2Base::drawPixel(int16_t x, int16_t y, uint8_t color)
// the above math can also be rewritten more simply as; // the above math can also be rewritten more simply as;
// row_offset = (y * WIDTH/8) & ~0b01111111 + (uint8_t)x; // row_offset = (y * WIDTH/8) & ~0b01111111 + (uint8_t)x;
// which is what the below assembler does // which is what the below assembler does
asm volatile( // local variable for the bitshift_left array pointer,
// which can be declared a read-write operand
const uint8_t* bsl = bitshift_left;
asm volatile
(
"mul %[width_offset], %A[y]\n" "mul %[width_offset], %A[y]\n"
"movw %[row_offset], r0\n" "movw %[row_offset], r0\n"
"andi %A[row_offset], 0x80\n" // row_offset &= (~0b01111111); "andi %A[row_offset], 0x80\n" // row_offset &= (~0b01111111);
@ -313,13 +318,14 @@ void Arduboy2Base::drawPixel(int16_t x, int16_t y, uint8_t color)
"adc r31, __zero_reg__\n" "adc r31, __zero_reg__\n"
// load correct bitshift from program RAM // load correct bitshift from program RAM
"lpm %[bit], Z\n" "lpm %[bit], Z\n"
: [row_offset] "=x&" (row_offset), // upper register (ANDI) : [row_offset] "=&x" (row_offset), // upper register (ANDI)
[bit] "=r" (bit), [bit] "=r" (bit),
[y] "+d" (y) // upper register (ANDI), must be writable [y] "+d" (y), // upper register (ANDI), must be writable
"+z" (bsl) // is modified to point to the proper shift array element
: [width_offset] "r" ((uint8_t)(WIDTH/8)), : [width_offset] "r" ((uint8_t)(WIDTH/8)),
[x] "r" ((uint8_t)x), [x] "r" ((uint8_t)x)
"z" (bitshift_left) :
:); );
if (color) { if (color) {
sBuffer[row_offset] |= bit; sBuffer[row_offset] |= bit;