GOSUB / RETURN

Syntax

Syntax: GOSUB label ... label: ... RETURN
Source: pb_cg_emit_gosub_return (pb_codegen.c:1738), pb_cg_emit_gosub_pop_return (pb_codegen.c:1768)

Description

Calls a local subroutine (defined by a label) within the same function and returns to the statement immediately following GOSUB. Unlike CALL (which invokes external DLL functions), GOSUB is an intra-procedure mechanism. The compiler maintains a dedicated GOSUB return address stack in the stack frame (max 32 entries). Each GOSUB pushes the return address; each RETURN pops and jumps to it.

Parameters

Parameter	Description
`label`	The name of the subroutine label to jump to. The subroutine must be in the same FUNCTION or SUB and must end with `RETURN`.
`RETURN`	Ends a GOSUB subroutine and returns execution to the statement after the calling `GOSUB`.

Example

FUNCTION PBMAIN() AS LONG
    PRINT "Main start"
    
    ' Call local subroutine
    GOSUB PrintBanner
    GOSUB PrintBanner
    
    PRINT "Main end"
    FUNCTION = 0
    
PrintBanner:
    PRINT "===================="
    PRINT "  PowerBASIC x64"
    PRINT "===================="
    PRINT
    RETURN
END FUNCTION

GOSUB - Push Return Address

mov    r10, [rbp - gosub_depth_offset]     ; current depth
cmp    r10, 32                             ; PB_GOSUB_STACK_MAX
jl     __pb_gosub_push_ok_N
mov    rcx, 1
call   [__imp_ExitProcess]                 ; overflow -> abort
__pb_gosub_push_ok_N:
lea    r11, [rbp - gosub_stack_base]       ; stack base
mov    rax, r10
imul   rax, 8                              ; depth * 8
sub    r11, rax                            ; r11 = &stack[depth]
lea    rax, [__pb_gosub_return_N]          ; return address
mov    [r11], rax                          ; store on stack
add    r10, 1
mov    [rbp - gosub_depth_offset], r10     ; depth++
jmp    __pb_label_<routine>_<label>        ; jump to subroutine

RETURN - Pop Return Address

mov    r10, [rbp - gosub_depth_offset]     ; current depth
sub    r10, 1
mov    [rbp - gosub_depth_offset], r10     ; depth--
lea    r11, [rbp - gosub_stack_base]
mov    rax, r10
imul   rax, 8
sub    r11, rax                            ; r11 = &stack[depth-1]
mov    rax, [r11]                          ; load return address
jmp    rax                                 ; indirect jump to return point
__pb_gosub_return_N:                       ; return target

Example Output

GOSUB SayHello
PRINT "Back from GOSUB"
FUNCTION = 0
SayHello:
PRINT "Hello!"
RETURN