call/ret

The call instructions push the program counter value of the next instruction onto the stack and set the program counter to the given address. The return instruction pops the program counter value and sets the program counter to it.

Prefixing these instructions with prefix_a16 results in undefined behavior. (The program counter value would be written to or read from IO devices.)

Instruction	Encoding	Semantics	Cycles
call imm16	00 001 001 imm16	see below	7
call pi	00 010 001	see below	5
ret	00 011 001	see below	4

Semantics of call imm16

Let $\mathit{PC}$ denote the address of the first byte after this instruction and $\mathit{pc}$ denote the 16 bit program counter register. $\mathit{sp}$ is the 16 bit stack pointer register.

$\begin{aligned} \mathit{sp} &\gets \mathit{sp} - 1; \\ \mathit{mem}[\mathit{sp}] &\gets \mathit{PC}[8:15]; \\ \mathit{sp} &\gets \mathit{sp} - 1; \\ \mathit{mem}[\mathit{sp}] &\gets \mathit{PC}[0:7]; \\ \mathit{pc} &\gets \mathit{imm16} \end{aligned}$

The microcode of this instruction uses the 16 bit shadow register $\mathit{uv}$ internally.

Semantics of call pi

Let $\mathit{PC}$ denote the address of the first byte after this instruction and $\mathit{pc}$ denote the 16 bit program counter register. $\mathit{sp}$ is the 16 bit stack pointer register and $\mathit{pi}$ is the 16 bit page-index register.

$\begin{aligned} \mathit{sp} &\gets \mathit{sp} - 1; \\ \mathit{mem}[\mathit{sp}] &\gets \mathit{PC}[8:15]; \\ \mathit{sp} &\gets \mathit{sp} - 1; \\ \mathit{mem}[\mathit{sp}] &\gets \mathit{PC}[0:7]; \\ \mathit{pc} &\gets \mathit{pi} \end{aligned}$

Semantics of ret

$\mathit{pc}$ denotes the 16 bit program counter register and $\mathit{sp}$ the 16 bit stack pointer register.

$\begin{aligned} \mathit{pc}[0:7] &\gets \mathit{mem}[\mathit{sp}]; \\ \mathit{sp} &\gets \mathit{sp} + 1; \\ \mathit{pc}[8:15] &\gets \mathit{mem}[\mathit{sp}]; \\ \mathit{sp} &\gets \mathit{sp} + 1 \end{aligned}$