2

I'm working on creating a processor definition for the Motorola / NXP HC11 using the Sleigh language for Ghidra. A definition for the HC12 already exists, so I'm working off of that.

The HC12 organizes its instructions into two pages: Page 1 with no prefix, and Page 2 with a prefix of 0x18:

HC12 Register Map

The HC11, on the other hand, has two more pages of registers:

HC11 Register Map

The HC12 processor definition uses a context variable and a recursive constructor to determine if an instruction resides on page 2:

define context contextreg
    Prefix18       = (0,0)    # 1 if 0x18 is the first byte
    PrefixHCS12X   = (0,0)    # 1 if first byte is 0x18 so that HCS12X to use GPAGE for memory access
    UseGPAGE       = (1,1)    # 1 if should use GPAGE concatenated to lower 16-bit EA
    XGATE          = (2,2)    # 1 if in xgate instruction decode mode
;

-

:^instruction  is XGATE=0 & op8=0x18; instruction  [ Prefix18=1; ] {}

-

:ABA                     is XGATE=0 & (Prefix18=1 & op8=0x06)
{
    result:1 = A + B;
    addition_flags1(A, B, result);
    A = result;
}

Another user, also defining an HC11 processor, just used 16-bit instructions instead:

define token withprebyte(16)
  pre   = (0,16);
define token byte(8)
  op8   = (0,7);

OFF: op8    is op8 { export *[const]:2 op8; }
IMM8: op8 is op8 { export *[const]:1 op8; }

:bsetY OFF,IMM8 is pre=0x181C; OFF; IMM8
{
  local temp = *:1 (Y+OFF);
  temp = temp | (IMM8);
  *:1 (Y+OFF) = temp;
}
:bclrY OFF,IMM8 is pre=0x181D; OFF; IMM8
{
  local temp = *:1 (Y+OFF);
  temp = temp & (~IMM8);
  *:1 (Y+OFF) = temp;
}

The approach used on the HC12 seems bizarre and complicated to me. I also don't see how it will work for more than one page. Is there a reason I shouldn't just use 16-bit tokens for those instructions which reside on other pages? Is using the context variable, as the HC12 implementation does, more elegant in some way that I'm not seeing?

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.