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PO
primarykey
Id
8290914
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0
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2011-11-28T02:32:53.107
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2011-11-28T02:32:53.107
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3583987
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8276172
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2
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1
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0
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text
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OK, I think I understand what is going on here. First, <code>const uint8_t* data</code> is a pointer to the data stored in PROGMEM. In the <code>function void drawString(uint16_t x, uint16_t y, uint16_t color, const FONT_INFO *fontInfo, char *str)</code> we pass a pointer to <code>fontInfo</code>. To continue, the following is important to understand; <pre><code>fontInfo.data (*ptr_to_fontInfo).data ptr_to_fontInfo->data </code></pre> are all the same. So <code>ptr_to_fontInfo->data</code> returns the data (not address) Then using the <code>&</code> operator, we pass the 'address of' this data to the next function <pre><code>drawCharBitmap(currentX, y, color, &fontInfo->data[charOffset], charWidth, fontInfo->height) </code></pre> This address is stored in the declared pointer variable <code>unint8_t *glyph</code> here; <pre><code>void drawCharBitmap(const uint16_t xPixel, const uint16_t yPixel, uint16_t color, const uint8_t *glyph, uint8_t cols, uint8_t rows) </code></pre> Keeping this in mind; <pre><code>int *ptr; int a; ptr = &a; </code></pre> Then glyph now points to the same address as <code>fontInfo->data[charOffset]</code>. The next thing to know is; <blockquote> a[b] in C is just a fancy way for writing *(a + b) </blockquote> So glyph being a pointer, when used like this <code>glyph[indexIntoGlyph]</code>, it is the same as <code>*(glyph + indexIntoGlyph)</code>, and the dereferencing operator <code>*</code> means we get the data at that address. From there, we can use the pgm rules as wex described; <blockquote> If the variable is in PROGMEM, you use pgm_read_byte() as a replacement for the dereference operator *. For "normal" variables in RAM you can always write *(&a) instead of just a to return the value of the variable a; so to return a 8-bit wide variable from progmem you write pgm_read_byte(&x). </blockquote> Hope this explanation is correct and helps people (newbies like me!) understand it a bit better.
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1. This table or related slice is empty.
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1. POLeaving a data array (Font) in FLASH - PROGMEM in AVR GCC
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1. PTAnswer
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1. USmriksman
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1. POLeaving a data array (Font) in FLASH - PROGMEM in AVR GCC
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1. VO
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