avr: use the correct RAM start address

Previously, the RAM was set to start at address 0. This is incorrect: on
AVR, the first few addresses are taken up by memory-mapped I/O. The
reason this didn't lead to problems (yet) was because the stack was
usually big enough to avoid real problems.

targets/avr.ld

@@ -2,7 +2,7 @@
 MEMORY
 {
     FLASH_TEXT (rw) : ORIGIN = 0,           LENGTH = __flash_size - _bootloader_size
-    RAM (xrw)       : ORIGIN = 0, LENGTH = __ram_size
+    RAM (xrw)       : ORIGIN = __ram_start, LENGTH = __ram_size
 }
 
 SECTIONS

tools/gen-device-avr/gen-device-avr.go

@@ -25,6 +25,7 @@ type AVRToolsDeviceFile struct {
 			Size           string `xml:"size,attr"`
 			MemorySegments []struct {
 				Name  string `xml:"name,attr"`
+				Start string `xml:"start,attr"`
 				Size  string `xml:"size,attr"`
 			} `xml:"memory-segment"`
 		} `xml:"address-spaces>address-space"`
@@ -57,9 +58,26 @@ type Device struct {
 	peripherals []*Peripheral
 }
 
+// AddressSpace is the Go version of an XML element like the following:
+//
+//     <address-space endianness="little" name="data" id="data" start="0x0000" size="0x0900">
+//
+// It describes one address space in an AVR microcontroller. One address space
+// may have multiple memory segments.
 type AddressSpace struct {
 	Size     string
-	Segments map[string]int
+	Segments map[string]MemorySegment
+}
+
+// MemorySegment is the Go version of an XML element like the following:
+//
+//     <memory-segment name="IRAM" start="0x0100" size="0x0800" type="ram" external="false"/>
+//
+// It describes a single contiguous area of memory in a particular address space
+// (see AddressSpace).
+type MemorySegment struct {
+	start int64
+	size  int64
 }
 
 type Interrupt struct {
@@ -115,14 +133,21 @@ func readATDF(path string) (*Device, error) {
 	for _, el := range device.AddressSpaces {
 		memorySizes[el.Name] = &AddressSpace{
 			Size:     el.Size,
-			Segments: make(map[string]int),
+			Segments: make(map[string]MemorySegment),
 		}
 		for _, segmentEl := range el.MemorySegments {
+			start, err := strconv.ParseInt(segmentEl.Start, 0, 32)
+			if err != nil {
+				return nil, err
+			}
 			size, err := strconv.ParseInt(segmentEl.Size, 0, 32)
 			if err != nil {
 				return nil, err
 			}
-			memorySizes[el.Name].Segments[segmentEl.Name] = int(size)
+			memorySizes[el.Name].Segments[segmentEl.Name] = MemorySegment{
+				start: start,
+				size:  size,
+			}
 		}
 	}
 
@@ -205,10 +230,12 @@ func readATDF(path string) (*Device, error) {
 		}
 	}
 
-	ramSize := 0 // for devices with no RAM
+	ramStart := int64(0)
+	ramSize := int64(0) // for devices with no RAM
 	for _, ramSegmentName := range []string{"IRAM", "INTERNAL_SRAM", "SRAM"} {
 		if segment, ok := memorySizes["data"].Segments[ramSegmentName]; ok {
-			ramSize = segment
+			ramStart = segment.start
+			ramSize = segment.size
 		}
 	}
 
@@ -227,6 +254,7 @@ func readATDF(path string) (*Device, error) {
 			"arch":             device.Architecture,
 			"family":           device.Family,
 			"flashSize":        int(flashSize),
+			"ramStart":         ramStart,
 			"ramSize":          ramSize,
 			"numInterrupts":    len(device.Interrupts),
 		},
@@ -429,6 +457,7 @@ func writeLD(outdir string, device *Device) error {
 /* Generated by gen-device-avr.go from {{.file}}, see {{.descriptorSource}} */
 
 __flash_size = 0x{{printf "%x" .flashSize}};
+__ram_start = 0x{{printf "%x" .ramStart}};
 __ram_size   = 0x{{printf "%x" .ramSize}};
 __num_isrs   = {{.numInterrupts}};
 `))