diff --git a/README.md b/README.md
index 83c1f58..1dc29bb 100644
--- a/README.md
+++ b/README.md
@@ -1,4 +1,45 @@
 
-# Minimal STM32 F4 task sheduler
+## Minimal STM32 F4/F7 task sheduler
 
-NanoOS: "It is impossible to do less" =)
+NanoOS: "Make less makes no sense" =)
+
+### Register frame description
+
+```
+/*
+ * ABI: r0..r3 are caller-saved (scratch registers), R4..R12 are callee-saved.
+ * It is appropriate to use R12 for a system call opcode (saved by NVIC). The
+ * stack pointer points to the current extent of the stack -- it is decremented
+ * before being used as index in a store. The stack grows downwards, to lower
+ * addresses. When an interrupt is processed, 8 registers are stored. LR is set
+ * to a special value that makes an ordinary function return into a return from
+ * interrupt. The LR value indicates which stack is going to be used (process
+ * or main) and can be modified before return.
+ *
+ *                  ____________________
+ *           Stack |                    |
+ *                 |                    |
+ *    higher       |        R4          | <-- SP saved in TCB (64B context)
+ *  addresses      |        R5          |   ^
+ *      |  ^       |        R6          |   |
+ *      |  |       |        R7          |   | 8 registers pushed by handler:
+ *      |  |       |        R8          |   | R4..R11
+ *      |  |       |        R9          |   | Full task context is now stored
+ *      V  |       |        R10         |   |
+ *         |       |        R11         |   |
+ *     direction   |        R0          | <-- SP when SVC handler gets control
+ *     of growth   |        R1          |   ^
+ *                 |        R2          |   |
+ *                 |        R3          |   | 8 registers are pushed by
+ *                 |        R12         |   | the NVIC hardware:
+ *                 |        LR (R14)    |   | xPSR, PC, LR, R12, R3..R0
+ *                 |        PC (R15)    |   |
+ *                 |       xPSR         |   |
+ *                 |                    | <-- SP before SVC
+ *                 |      (stuff)       |
+ *       Stack +   |                    |
+ *       StackSize |____________________|
+ *
+ */
+
+```