@ vim:ft=armv4
@ assume SP starts at 200
@ expected result: 0x200: 0xff00, 0x204: 0xff80, 0x208: 0x7fffff80

.thumb
mov r7, sp
mov r0, #0xff
mov r6, #0
strb r0, [r7, #1]
ldrsh r1, [r7, r6]
lsr r1, r1, #1
strh r1, [r7, #4]
str r1, [sp, #8]