x86 asm: move jcc from x86-assembly-cheat

README.adoc

@@ -12382,6 +12382,31 @@ Bibliography:
 * link:userland/arch/x86_64/jmp.S[JMP]
 ** link:userland/arch/x86_64/jmp_indirect.S[JMP indirect]
 
+==== x86 Jcc instructions
+
+link:userland/arch/x86_64/jcc.S[Jcc]
+
+Jump if certain conditions of the flags register are met.
+
+Jcc includes the instructions:
+
+* JZ, JNZ
+** JE, JNE: same as JZ, with two separate manual entries that say almost the same thing, lol: https://stackoverflow.com/questions/14267081/difference-between-je-jne-and-jz-jnz/14267662#14267662
+* JG: greater than, signed
+** JA: Above: greater than, unsigned
+* JL: less than, signed
+** JB below: less than, unsigned
+* JC: carry
+* JO: overflow
+* JP: parity. Why it exists: https://stackoverflow.com/questions/25707130/what-is-the-purpose-of-the-parity-flag-on-a-cpu
+* JPE: parity even
+* JPO: parity odd
+
+JG vs JA and JL vs JB:
+
+* https://stackoverflow.com/questions/20906639/difference-between-ja-and-jg-in-assembly
+* https://stackoverflow.com/questions/9617877/assembly-jg-jnle-jl-jnge-after-cmp
+
 === x86 SIMD
 
 History:

userland/arch/x86_64/jcc.S

@@ -0,0 +1,62 @@
+/* https://github.com/cirosantilli/linux-kernel-module-cheat#x86-jcc-instructions */
+
+#include <lkmc.h>
+
+LKMC_PROLOGUE
+    /* JZ, JE */
+    mov $1, %r12
+    cmp $1, %r12
+    LKMC_ASSERT(jz)
+    cmp $2, %r12
+    LKMC_ASSERT(jnz)
+
+    /* JA, JB, JG, JL */
+
+        /* 0x0 ==
+         *
+         * * 0 in 2's complement signed
+         * * 0 in 2's complement unsigned
+         */
+        mov $0, %al
+
+        /* 0xFF ==
+         *
+         * *  -1 in 2's complement signed
+         * * 255 in 2's complement unsigned
+         */
+        mov $0xFF, %bl
+
+        /* Do the operation. */
+        cmp %bl, %al
+
+        /* We push and pop flags with PUSHF and POPF because
+         * our assert function might change them. All comparisons
+         * are done with the flags of the original cmp operation.
+         *
+         * We push twice to keep the stack aligned to 16 bits
+         * when calling our C asset function.
+         */
+        pushf
+        pushf
+
+        /* 0 < 255 */
+        LKMC_ASSERT(jb)
+        popf
+        pushf
+
+        /* ! 0 > 255 */
+        LKMC_ASSERT(jna)
+        popf
+        pushf
+
+        /* ! 0 < -1 */
+        LKMC_ASSERT(jnl)
+        popf
+        pushf
+
+        /* 0 > -1 */
+        LKMC_ASSERT(jg)
+
+        /* Restore stack. */
+        add $16, %rsp
+LKMC_EPILOGUE