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Embedded AVR - read strings from Flash

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Timm Thaler:
Programming AVR controllers with Freepascal I try to read strings from flash.

In ASM I put the z-pointer to the string address in flash and read by LPM instruction. In AVR-GCC I use PSTR("MyString") and pgm_read_byte(address) to tell the compiler to read from flash.

In Freepascal I tried with

--- Code: Text  [+][-]window.onload = function(){var x1 = document.getElementById("main_content_section"); if (x1) { var x = document.getElementsByClassName("geshi");for (var i = 0; i < x.length; i++) { x[i].style.maxHeight='none'; x[i].style.height = Math.min(x[i].clientHeight+15,306)+'px'; x[i].style.resize = "vertical";}};} ---  buffer : array[0..16] of char;  buffer := pchar('MyStringMyString');  for i := 0 to 15 do    sendchar(buffer[i]);
This runs, I can put strings to the buffer und send the chars.

But the strings are not read from flash but from ram:


--- Code: ASM  [+][-]window.onload = function(){var x1 = document.getElementById("main_content_section"); if (x1) { var x = document.getElementsByClassName("geshi");for (var i = 0; i < x.length; i++) { x[i].style.maxHeight='none'; x[i].style.height = Math.min(x[i].clientHeight+15,306)+'px'; x[i].style.resize = "vertical";}};} ---        ldi     r19,lo8(_sGH_DISPLAYs_Ld3)        ldi     r18,hi8(_sGH_DISPLAYs_Ld3)  <= get ram address        ldi     r26,17  <= count chars        mov     r20,r26        mov     r30,r19        mov     r31,r18  <= ram address to z-pointer        ldi     r18,lo8(U_sGH_DEFINE_ss_BUFFER)        ldi     r19,hi8(U_sGH_DEFINE_ss_BUFFER) <= buffer address        movw    r26,r18  <= buffer address to x-pointer.Lj33:        ld      r0,Z+  <= read from ram        st      X+,r0  <= write to buffer        dec     r20        brne    .Lj33
So I checked the complete hexfile by disassembling and found:


--- Code: ASM  [+][-]window.onload = function(){var x1 = document.getElementById("main_content_section"); if (x1) { var x = document.getElementsByClassName("geshi");for (var i = 0; i < x.length; i++) { x[i].style.maxHeight='none'; x[i].style.height = Math.min(x[i].clientHeight+15,306)+'px'; x[i].style.resize = "vertical";}};} ---  72:   a0 e0           ldi     r26, 0x00       ; 0  74:   b1 e0           ldi     r27, 0x01       ; 1  76:   d1 e0           ldi     r29, 0x01       ; 1  78:   ee ee           ldi     r30, 0xEE       ; 238  <= z-pointer to flash address, strings start here  7a:   f8 e0           ldi     r31, 0x08       ; 8  7c:   02 c0           rjmp    .+4             ;  0x82  7e:   05 90           lpm     r0, Z+  <= read chars from flash  80:   0d 92           st      X+, r0  <= store chars in ram  82:   a6 34           cpi     r26, 0x46       ; 70  84:   bd 07           cpc     r27, r29  86:   d9 f7           brne    .-10            ;  0x7e
So at first all string are copied from flash to ram and then single strings are copied from ram to buffer (also ram). As the ram in the AVR controllers is limited this is no good idea.

Is it possible to tell the compiler to copy strings from flash to buffer directly? Or can I access flash data without copying it to ram?

Sorry, there is not much information about programming AVR controllers with Freepascal out there.

Timm Thaler:
I also tried with strpcopy but this seems not to be available for embedded AVR.

ccrause:
I assume you are referring to the use of __flash or PROGMEM attribute in e.g. GCC?  I suspect that is not yet possible in FPC.

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