From: jl@glen--canyon.com   
      
   On Thu, 12 Feb 2026 21:05:53 +0100, Lasse Langwadt    
   wrote:   
      
   >On 2/12/26 19:19, john larkin wrote:   
   >> The internal stack-oriented architecture of Focal-8 inspired the cool   
   >> PDP-11 architecture which in turn inspired the 68K.   
   >   
   >how did stack-oriented inspire architectures with lots of orthogonal   
   >registers, by showing how it should not be done?   
   >   
      
   Stack-oriented inspired stack-oriented. In the PDP-11, any of the   
   registers could be used as a stack pointer.   
      
   Focal-8 faked the stack operations with klunky PDP-8 opcodes. The 8's   
   subroutine call machanism was barbaric: save the return addess at the   
   first word of the destination page. Rick invented PUSHJ, save the RA   
   on the stack and jump anywhere in the 4K space.   
   He made the PDP-8 into a sort of RPN machine.   
      
   DEC was going to build yet another clumsy 16-bit computer, like the   
   NOVA or the HP 2114 thing, but Rick talked them into the elegant   
   PDP-11 architecture, a stack-oriented machine with eight registers.   
      
   It could do fun things like   
      
   NEG PC   
      
   negate the program counter.   
      
   There was the land mine opcode   
      
   MOV -(PC), -(PC)   
      
   (014747 octal)   
      
   which copied itself one location below itself and re-executed that.   
      
      
   John Larkin   
   Highland Tech Glen Canyon Design Center   
   Lunatic Fringe Electronics   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)