eei/virtualeei: implement io.WriterAt

cmd/riscvrun/main.go

@@ -26,7 +26,10 @@ func main() {
 	if err != nil {
 		panic(err)
 	}
-	copy(sw.Ram[*loadAddress:*loadAddress+len(fb)], fb)
+	_, err = sw.WriteAt(fb, *loadAddress)
+	if err != nil {
+		panic(err)
+	}
 
 	c := riscvemu.NewCPU(&sw)
 	c.Pc = uint32(*loadAddress)

virtualEei.go

@@ -63,6 +63,25 @@ func (ve *VirtualEEI) Write32(addr, value uint32) error {
 	return nil
 }
 
+// WriteAt implements the io.WriterAt interface on top of the EEI's virtual memory.
+func (ve *VirtualEEI) WriteAt(p []byte, off int64) (n int, err error) {
+	if off < 0 || off >= math.MaxUint32 {
+		return 0, os.ErrInvalid
+	}
+
+	addr := uint32(off) // bounds-checked
+
+	// TODO do something more optimized
+	for i, bb := range p {
+		err := ve.WriteByte(addr+uint32(i), bb)
+		if err != nil {
+			return i, err
+		}
+	}
+
+	return len(p), nil
+}
+
 func (ve *VirtualEEI) Syscall() {
 	panic("syscall")
 }