+# Overview #
+
+Although GCC claims to support PDP-11 targets there are some bugs that must be
+worked around both in GCC and the GNU assembler.
+
+# GNU binutils Bugs #
+
+## Problem Description ##
+
+One of the addressing modes supported by the PDP-11 is 'index deferred',
+represented by `@X(Rn)`. This operand indicates that `Rn` contains a pointer
+which should be dereferenced and the result added to `X` to generate a new
+pointer to the final location. For example, consider the following four values,
+one stored in a register and the other three in memory. Then `@2(R1)` is the
+value `222`.
+
+ R1: 1000
+ 1000: 2000
+ 2000: 111
+ 2002: 222
+
+Similarly, `@0(R1)` is the value `111`. In most PDP-11 assemblers, including
+DEC's MACRO-11 assembler, the string `@(Rn)` is an alias to `@0(Rn)`. But when
+the GNU assembler encounters `@(Rn)` it assembles it as though it were `(Rn)`,
+a single level of indirection instead of two levels!
+
+If we're only writing assembly then we can work around this bug by always using
+the form `@0(Rn)`. But what if we're writing C and using GCC to compile it?
+Consider the following C code example, taken directly from some stack-based
+debugger code written for the PDP-11.
+
+ uint16_t ** csp = (uint16_t **) 070000;
+ *csp = (uint16_t *) 060000;
+ **csp = 0;
+
+When GCC transpiles this to assembly it generates code of the form `@(Rn)` when
+assigning a value to `**csp` thus causing the value `0` to overwrite the value
+`060000` at `*csp` if GNU `as` is used to assemble the code.
+
+## Solution Description ##
+
+The following patch, tested on GNU binutils 2.28, fixes the bug. Since it
+overloads the `operand->code` variable to pass unrelated state information to
+`parse_reg()` I haven't submitted it for inclusion in GNU binutils. Once I'm
+done bug-hunting in my toolchain I will clean up all the fixes and submit them
+in one bundle.
+
+ --- tc-pdp11.c 2017-06-24 22:33:00.260210000 -0700
+ +++ tc-pdp11.c.fixed 2017-06-24 22:32:12.455205000 -0700
+ @@ -431,6 +431,9 @@
+ {
+ LITTLENUM_TYPE literal_float[2];
+
+ + /* Store the value (if any) passed by parse_op_noreg() before parse_reg() overwrites it. */
+ + int deferred = operand->code;
+ +
+ str = skip_whitespace (str);
+
+ switch (*str)
+ @@ -451,6 +454,15 @@
+ operand->code |= 020;
+ str++;
+ }
+ + /*
+ + * This catches the case where @(Rn) is interpreted as (Rn) rather than @0(Rn)
+ + */
+ + else if (deferred)
+ + {
+ + operand->additional = 1;
+ + operand->word = 0;
+ + operand->code |= 060;
+ + }
+ else
+ {
+ operand->code |= 010;
+ @@ -581,6 +593,12 @@
+
+ if (*str == '@' || *str == '*')
+ {
+ + /*
+ + * operand->code is overwritten by parse_reg() inside parse_op_no_deferred()
+ + * We use it to temporarily catch the alias @(Rn) -> @0(Rn) since
+ + * parse_op_no_deferred() starts at str+1 and thus misses the '@'.
+ + */
+ + operand->code |= 010;
+ str = parse_op_no_deferred (str + 1, operand);
+ if (operand->error)
+ return str;
projects / website_subgeniuskitty.com / commitdiff