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PUSHJ C(AC)<-C(AC)+<1,,1>; C(CR(AC))<-<flags,,PC>; PC<-E;PUSHJ (PUSH return address and Jump) is like PUSH except the data that is pushed onto the top of the stack is the PC and flags word. The PC that is stored is the PC of the instruction that follows the PUSHJ. Then the PC is set to the effective address of the instruction. Pushdown overflow results if the AC becomes positive when it is incremented.
POPJ PC<-CR(CR(AC)); C(AC)<-C(AC)-<1,,1>POPJ (POP return address and Jump) undoes PUSHJ. The right half of the word at the top of the stack is loaded into the PC (the flags are unchanged). Then the stack pointer is decremented as in POP. The effective address of POPJ is ignored. Pushdown overflow obtains if the AC becomes negative as a result of the subtraction.
If a subroutine called by PUSHJ AC, wants to skip over the instruction following the PUSHJ, the following sequence accomplishes that result:
AOS (AC) ;AC better be non zero. POPJ AC,If you must restore the flags that PUSHJ saved, the following sequence should be used instead of POPJ:
POP AC,(AC) ;Adjust the stack JRST 2,@1(AC) ;Restore flags and PC from old stack top.However, this sequence has a timing error in that the word is released from the stack while its contents are still needed. This can cause a bug if you have any interrupt processing in your program. JRST, Jump and ReSTore, is an unconditional jump instruction. In JRST, the AC field does not address an accumulator. Instead, the AC is decoded to signify various things.
JRST PC<-E; JRST 2, PC<-E; flags are restored (see text); JRST 10, PC<-E; Dismiss current priority interrupt; JRST 12, PC<-E; restore flags and dismiss priority interrupt;If the AC field is zero, only a jump occurs. JRST is everyone's favorite unconditional jump instruction (the only other one is JUMPA which is more typing, also, on the KA-10 JUMPA is slower than JRST).
JRST 2, (i.e., JRST with AC field set to 2) signifies jump and restore flags. (The assembler also recognizes the mnemonic JRSTF for JRST 2,). If indirection is used in JRSTF, then the flags are restored from the last word fetched in the address calculation. If indexing is used with no indirection, the flags are restored from the left half of the specified index register. If neither indexing nor indirection is used in the address calculation the flags are restored from the left half of the JRSTF itself! In a user mode program JRSTF cannot clear USER nor can it set IOT User (it can however, clear IOT User).
The following are all illegal in user mode and are trapped as UUOs.
JRST 4, (alternate mnemonic HALT) sets the PC from E and stops the processor.
JRST 10, is used to dismiss the current priority interrupt. Usually JRST 12, is used for this purpose since JRST 10, fails to retore flags.
JRST 12, (an alternate mnemonic is JEN, jump and enable priority interrupts) combines the functions of JRST 10, and JRST 2,. The JFCL (Jump on Flag and CLear) instruction is another case in which the AC field is decoded to modify the instruction. The AC field selects the four flags in PC bits 0 through 3. PC bits 0 to 3 correspond to bits 9 to 12 in the JFCL instruction. JFCL will jump if any flag selected by the AC field is a 1. All flags selected by the AC field are set to zero.
JFCL 0, since it selects no PC bits, is a no-op.
JFCL 17, will clear all flags, and will jump if any of AROV, CRY0, CRY1, or FOV are set.
JFCL 1, (JFOV) jumps if FOV is set and clears FOV.
JFCL 10, (JOV) jumps if AROV is set and clears AROV. XCT, the execute instruction, fetches the word addressed by the effective address and executes that word as an instruction. In the case of XCTing an instruction that stores a PC, the PC that is stored is the address of the instruction that follows the XCT. If the executed instruction skips, then that skip is relative to the XCT. The AC field of the XCT should be zero. [In exec mode a non zero AC field in an XCT is significant.]