-# 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;
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