Mechanism to protect data saved on a local register cache during inter-subsystem calls and returns

Claim 1

A method of protecting intersubsystem calls and returns that cross subsystem boundaries comprising steps of: A. Detecting a "set boundary bit" instruction as part of a first instruction flow for an inter-subsystem call; B. Setting a left most bit of a shift register to a 1 upon a first condition that said "set boundary bit" instruction is detected; C. Detecting a first instruction signifying an intra-subsystem call; D. Asserting a PUSH line in response to said detecting said first instruction; E. Shifting said shift register right one bit upon a second condition that said PUSH line is asserted; F. Detecting a second instruction signifying an intra-subsystem return; G. Asserting a POPTO2 line in response to said detecting said second instruction; H. Checking said leftmost bit of said shift register for a zero; I. Signaling a protection fault upon a condition that said leftmost bit is not zero; and, J. Shifting said register shift left one bit upon a condition that a third instruction following said second instruction is detected.

Claim 2

A method of protecting intersubsystem calls and returns that cross subsystem boundaries comprising steps of: A. Detecting a "set boundary bit" instruction as part of a first instruction flow for an inter-subsystem call; B. Setting a left most bit of a shift register to a 1 upon a first condition that said "set boundary bit" instruction is detected; C. Detecting a first instruction signifying an intra-subsystem call; D. Asserting a PUSH line in response to said detecting said first instruction; E. Shifting said shift register right one bit upon a second condition that said PUSH line is asserted; F. Detecting a second instruction signifying an intra-subsystem return; G. Asserting a POPTO2 line in response to said detecting said second instruction; H. Checking said leftmost bit of said shift register for a zero; I. Signaling a protection fault upon a condition that said leftmost bit is not zero; and, J. Shifting said register shift left one bit upon a condition that a third instruction following said second instruction is detected.

Claim 4

An apparatus for protecting intersubsystem calls and returns that cross subsystem boundaries comprising: a shift register; first decoder means connected to said shift register for detecting a "set boundary bit" instruction as part of the flow for an inter-subsystem call; second means connected to said first decoder means and to said shift register for setting a left most bit of said shift register to a 1 if said "set boundary bit" instruction is detected; multiple decoder means for detecting an intra-subsytem call instruction, an intra-subsytem return instruction, and a first instruction following said intra-subsytem return instruction; a PUSH line connected between said multiple decoder means and said shift register; a POPTOS2 line connected between said multiple decoder means decoder and said shift register; right shift means connected to said multiple decoder means for shifting said shift register right one bit upon detection of said intra-subsytem call instruction; zero bit comparison means, connected to said shift register and to said multiple decoder means, for checking said leftmost bit of said shift register for a zero; inter-subsytem return fault detection means connected to said zero bit comparison means for signaling a protection fault upon a first condition that said leftmost bit is not zero concurrently with a detection of said inter-subsystem return instruction by said multiple decoder means; and, left shift means connected to said multiple decoder means for shifting said register shift left one bit upon detection of said first instruction following said intra-subsytem return instruction.

Claim 5

An apparatus for protecting intersubsystem calls and returns that cross subsystem boundaries comprising: a shift register; first decoder means connected to said shift register for detecting a "set boundary bit" instruction as part of the flow for an inter-subsystem call; second means connected to said first decoder means and to said shift register for setting a left most bit of said shift register to a 1 if said "set boundary bit" instruction is detected; multiple decoder means for detecting an intra-subsytem call instruction, an intra-subsytem return instruction, and a first instruction following said intra-subsytem return instruction; a PUSH line connected between said multiple decoder means and said shift register; a POPTOS2 line connected between said multiple decoder means decoder and said shift register; right shift means connected to said multiple decoder means for shifting said shift register right one bit upon detection of said intra-subsytem call instruction; zero bit comparison means, connected to said shift register and to said multiple decoder means, for checking said leftmost bit of said shift register for a zero; inter-subsytem return fault detection means connected to said zero bit comparison means for signaling a protection fault upon a first condition that said leftmost bit is not zero concurrently with a detection of said inter-subsystem return instruction by said multiple decoder means; and, left shift means connected to said multiple decoder means for shifting said register shift left one bit upon detection of said first instruction following said intra-subsytem return instruction.

Claim 7

An apparatus for protecting inter subsystem calls and returns that cross subsystem boundaries comprising: a subsystem call subsystem return decoder; a shift register connected to said subsystem call subsystem return decoder; a PUSH line connected between said subsystem call subsystem return decoder and said shift register; a POPTOS2 line connected between said subsystem call subsystem return decoder and said shift register; said subsystem call subsystem return decoder first logic that asserts said PUSH line upon a decoding of a subsystem call instruction; said subsystem call subsystem return decoder including second logic that asserts said POPTOS2 line upon a decoding of a subsystem return instruction; a set boundary bit decoder; a stcp0 line connected between said set boundary bit decoder and said shift register; said set boundary bit decoder including a third logic that detects a set boundary bit instruction as part of a flow for an inter-subsystem call and asserts said stcp0 line in response a detection of said set boundary bit instruction; assertion of said stcp0 line causing a leftmost bit of said shift register to be set; assertion of said PUSH line causing said shift register to shift right one bit position; assertion of said POPTOS2 line causing said shift register to shift left one bit position; said subsystem call subsystem return decoder including a fourth logic that asserts said POPTOS2 line upon a decoding of said POP instruction; said subsystem call subsystem return decoder including a fifth logic that asserts said PUSH line upon a decoding of said PUSH instruction; a bit 0 line output from said shift register; said bit 0 line being asserted upon a first condition that said 0 bit of said shift register is a 0 and being unasserted upon a second condition that said 0 bit of said shift register is a 1; and, a comparison logic connected to said bit 0 line, to said POPTOS2 line and to said PUSH line; said comparison logic including a fault line output; said comparing logic comparing said POPTOS2 line and said PUSH line with said bit 0 line, said fault line being asserted upon a condition that said POPTOS2 line is asserted and said bit 0 line is not asserted and upon a condition that said PUSH line is asserted and said bit 0 line is asserted.

Claim 8

An apparatus for protecting inter subsystem calls and returns that cross subsystem boundaries comprising: a subsystem call subsystem return decoder; a shift register connected to said subsystem call subsystem return decoder; a PUSH line connected between said subsystem call subsystem return decoder and said shift register; a POPTOS2 line connected between said subsystem call subsystem return decoder and said shift register; said subsystem call subsystem return decoder first logic that asserts said PUSH line upon a decoding of a subsystem call instruction; said subsystem call subsystem return decoder including second logic that asserts said POPTOS2 line upon a decoding of a subsystem return instruction; a set boundary bit decoder; a stcp0 line connected between said set boundary bit decoder and said shift register; said set boundary bit decoder including a third logic that detects a set boundary bit instruction as part of a flow for an inter-subsystem call and asserts said stcp0 line in response a detection of said set boundary bit instruction; assertion of said stcp0 line causing a leftmost bit of said shift register to be set; assertion of said PUSH line causing said shift register to shift right one bit position; assertion of said POPTOS2 line causing said shift register to shift left one bit position; said subsystem call subsystem return decoder including a fourth logic that asserts said POPTOS2 line upon a decoding of said POP instruction; said subsystem call subsystem return decoder including a fifth logic that asserts said PUSH line upon a decoding of said PUSH instruction; a bit 0 line output from said shift register; said bit 0 line being asserted upon a first condition that said 0 bit of said shift register is a 0 and being unasserted upon a second condition that said 0 bit of said shift register is a 1; and, a comparison logic connected to said bit 0 line, to said POPTOS2 line and to said PUSH line; said comparison logic including a fault line output; said comparing logic comparing said POPTOS2 line and said PUSH line with said bit 0 line, said fault line being asserted upon a condition that said POPTOS2 line is asserted and said bit 0 line is not asserted and upon a condition that said PUSH line is asserted and said bit 0 line is asserted.