machine: add flash start/end address that can be used for data

src/examples/flash/main.go

@@ -0,0 +1,8 @@
+package main
+
+import "machine"
+
+func main() {
+	println("flash data start:", machine.FlashDataStart())
+	println("flash data end:  ", machine.FlashDataEnd())
+}

src/machine/flash.go

@@ -0,0 +1,23 @@
+//go:build nrf
+
+package machine
+
+import "unsafe"
+
+//go:extern __flash_data_start
+var flashDataStart [0]byte
+
+//go:extern __flash_data_end
+var flashDataEnd [0]byte
+
+// Return the start of the writable flash area, aligned on a page boundary. This
+// is usually just after the program and static data.
+func FlashDataStart() uintptr {
+	return (uintptr(unsafe.Pointer(&flashDataStart)) + flashPageSize - 1) &^ (flashPageSize - 1)
+}
+
+// Return the end of the writable flash area. Usually this is the address one
+// past the end of the on-chip flash.
+func FlashDataEnd() uintptr {
+	return uintptr(unsafe.Pointer(&flashDataEnd))
+}

src/machine/machine_nrf51.go

@@ -6,6 +6,8 @@ import (
 	"device/nrf"
 )
 
+const flashPageSize = 1024
+
 // Get peripheral and pin number for this GPIO pin.
 func (p Pin) getPortPin() (*nrf.GPIO_Type, uint32) {
 	return nrf.GPIO, uint32(p)

src/machine/machine_nrf528xx.go

@@ -12,6 +12,8 @@ func CPUFrequency() uint32 {
 	return 64000000
 }
 
+const flashPageSize = 4096
+
 // InitADC initializes the registers needed for ADC.
 func InitADC() {
 	return // no specific setup on nrf52 machine.

targets/arm.ld

@@ -69,3 +69,7 @@ _heap_start = _ebss;
 _heap_end = ORIGIN(RAM) + LENGTH(RAM);
 _globals_start = _sdata;
 _globals_end = _ebss;
+
+/* For the flash API */
+__flash_data_start = LOADADDR(.data) + SIZEOF(.data);
+__flash_data_end = ORIGIN(FLASH_TEXT) + LENGTH(FLASH_TEXT);