Merge pull request #241 from kito-cheng/medlow-range

Clarify the range of medlow code model on rv64

Author: kito-cheng

riscv-elf.adoc

@@ -21,7 +21,8 @@ in the current code model.
 === Small code model
 
 The small code model, or `medlow`, allows the code to address the whole RV32
-address space or the lower 2 GiB of the RV64 address space.
+address space or the lower 2 GiB and highest 2GiB of the RV64 address space
+(`0xFFFFFFFF7FFFF800` ~ `0xFFFFFFFFFFFFFFFF` and `0x0` ~ `0x000000007FFFF7FF`).
 By using the instructions `lui` and `ld` or `st`, when referring to an object, or
 `addi`, when calculating an address literal, for example,
 a 32-bit address literal can be produced.
@@ -44,6 +45,20 @@ an address in the `medlow` code model.
     addi a0, a0, %lo(symbol)
 ----
 
+NOTE: The ranges on RV64 are not `0x0` ~ `0x000000007FFFFFFF` and
+`0xFFFFFFFF80000000` ~ `0xFFFFFFFFFFFFFFFF` due to RISC-V's sign-extension of
+immediates; the following code fragments show where the ranges come from:
+[,asm]
+----
+# Largest postive number:
+lui a0, 0x7ffff # a0 = 0x7ffff000
+addi a0, 0x7ff # a0 = a0 + 2047 = 0x000000007FFFF7FF
+
+# Smallest negative number:
+lui a0, 0x80000 # a0 = 0xffffffff80000000
+addi a0, a0, -0x800 # a0 = a0 + -2048 = 0xFFFFFFFF7FFFF800
+----
+
 === Medium code model
 
 The medium code model, or `medany`, allows the code to address the range