implement u-format for JAL

4 files changed, 29 insertions, 10 deletions

diff --git a/elf.py b/elf.py
index 8990c88..919b690 100755
--- a/elf.py
+++ b/elf.py
@@ -1,6 +1,6 @@
 #!/usr/bin/env python3
 import sys
-from subx import clean, classify, parse_segment, parse_instr, format_instr
+from subx import clean, classify, parse_segment, parse_instr
 
 """
 Takes a packed & surveyed hex stream (with section headers) and packs it into a
@@ -28,16 +28,14 @@ for line in sys.stdin:
     (name, addr) = parse_segment(line)
     segment = { 'name': name, 'addr': addr, 'content': [] }
     segments.append(segment)
-  else:
+  elif type == 'instr':
     if segment == None:
       raise ValueError("label or code outside of segment!")
 
-    if type == 'label':
-      raise NotImplementedError()
-    else:
-      instr = parse_instr(line)
-      segment['content'] += instr
-
+    instr = parse_instr(line)
+    segment['content'] += instr
+  else:
+    raise ValueError("elf input should contain only segments and instructions!")
 entrypoint = 0x80000000
 cursor = 0
 
diff --git a/notes.md b/notes.md
index f3e80bc..4351b27 100644
--- a/notes.md
+++ b/notes.md
@@ -117,7 +117,7 @@ opcode: 0b0010111 / 0x17
 - imm20: top 20 bits of offset added to PC
 - result stored in rd
 
-## UJ-Format: op(imm20/21) -> rd
+## U-Format: op(imm20/21) -> rd
 `opcode[7] rd[5] imm31:12[20]`
 immediates on a 32bit scale (jal)
 
diff --git a/riscv.py b/riscv.py
index b2af4cc..c0d5a15 100644
--- a/riscv.py
+++ b/riscv.py
@@ -29,3 +29,16 @@ def format_u(op, rd, imm20):
   b2 = (imm20 >> 4) & 0xff
   b3 =  imm20 >> 12
   return [(b0,), (b1,), (b2,), (b3,)]
+
+def format_j(op, rd, imm):
+  # opcode[7] rd[5] imm19:12[8] imm11[1] imm10:1[10] imm20[1]
+  imm12 = (imm >> 11) & 0xff
+  imm11 = (imm >> 12) & 0x1
+  imm1 = imm & 0x3ff
+  imm20 = imm >> 19
+
+  b0 =    (rd & 0x1) << 7 | op      # rd[1] op[7:0]
+  b1 = (imm12 & 0xf) << 4 | rd >> 1 # imm[15:12] rd[5:2]
+  b2 = (imm1 << 5) & 0xe0 | imm11 << 4 | (imm12 >> 4)          # imm[3:1] imm[11] imm[19:16]
+  b3 = imm20 << 7 | imm1 >> 3
+  return [(b0,), (b1,), (b2,), (b3,)]
diff --git a/test.py b/test.py
index ba6ffd6..987535f 100755
--- a/test.py
+++ b/test.py
@@ -1,7 +1,13 @@
 #!/usr/bin/env python3
-from riscv import format_u, format_i, format_s
+from riscv import format_u, format_i, format_s, format_j
 from subx import format_instr
 
+def neg(val, bits):
+  """compute the 2's complement of int value val"""
+  # if (val & (1 << (bits - 1))) != 0: # if sign bit is set e.g., 8bit: 128-255
+  val = val - (1 << bits)        # compute negative value
+  return -val         
+
 if __name__ == "__main__":
   t0 = 0x5
   t1 = 0x6
@@ -19,3 +25,5 @@ if __name__ == "__main__":
   print(format_instr(format_s(0x23, t0, t1, 0, 0x2), "sw t1, 0(t0)"))
   print(format_instr(format_i(0x13, t1, 0, 10, 0x0), "addi t1, x0, 10"))
   print(format_instr(format_s(0x23, t0, t1, 0, 0x2), "sw t1, 0(t0)"))
+  print(format_instr(format_j(0x6f, 0, neg(26, 20)), "jal x0, -26"))
+