Data processing system write protection mechanism

Claim 1

A microprogram-controlled data processing system comprising: a source of a plurality of user-generated control words; a control store having a first control memory means and a second control memory means, each said control memory means including a plurality of storage locations for addressably storing said user-generated control words; means for transferring in seriatim said plurality of user-generated control words from said source to said control store; a data input register in said control store for receiving said transferred user-generated control words; address generating means in said control store for generating addresses associated with said storage locations in said first control memory means and said second control memory means; write control means for controlling the transfer of each of said user-generated control words received by said data input register from said data input register into one of said storage locations associated with said generated addresses in said first control memory means and said second control memory means to load said user-generated control words into said first control memory means and said second control memory means; a third control memory means for addressably storing system control words different from said user-generated control words; a central processing unit coupled to said source, said control store, and said third control memory means, said central processing unit being constructed and arranged to execute either a said system control word or a said user-generated control word under said microprogram-control, said central processor unit for initiating the transfer of said user-generated control words from said source to said control store over said transferring means; means for generating a write signal in response to said transfer initiation by said central processing unit to indicate that said user-generated control words are being loaded into said first and second control memory means; means for selectively transferring to said central processing unit for execution thereby said user-generated control words from said first control memory means and said second control memory means, and said system control words from said third control memory means; and means responsive to said write signal for inhibiting the transfer of said user-generated control words from said first control memory means and said second control memory means to said central processing unit while said user-generated control words are being loaded into said first control memory means and said second control memory means.